Add community Windows GUI configurator - #76

Summary

This PR contributes a community-maintained Windows GUI configurator for MouseMaster
properties files, developed in the Rezetyan/mousemaster
fork and already released there (most recently as
configurator v2.1.0).

Everything lives in a self-contained configurator/ directory: C# WinForms source, the
build script, an English README (中文说明), an MIT license
for the new code, and two layers of tests. There are no changes to any existing
MouseMaster file beyond a short README section and .gitignore entries for the build
artifacts.

What it does

• Bilingual UI (Simplified Chinese / English) following the Windows display language,
  switchable at runtime; the choice is remembered in the registry.
• Rebinds 37 actions in six categories (activation, movement, mouse buttons, wheel,
  mode switches, hints, automation) plus four motion values (mouse/wheel max velocity
  and acceleration). Only the default branch of the dynamic velocity properties is
  replaced; modifier branches and initial velocities are preserved.
• Exact-chord conflict resolution: rebinding to an already-used shortcut in the same
  mode scope clears the old binding and reports the change.
• Careful combo generation: normal-mode entry combos are generated with mutual
  same-key modifier exclusion (^{...}), so a modifier-free binding can never shadow a
  modified one (e.g. Screen Hint Ctrl+F vs UI Hint F generate _{leftctrl} +f and
  ^{leftctrl} +f instead of both firing on Ctrl+F).
• Optional focus mode: generates per-mode swallow-all fallback aliases for the eight
  keyboard-mouse modes, excluding keys used by each mode's existing non-swallowing
  combos, and comments out OS-bound macros while it is on.
• Alt-Tab auto-centering can be toggled and its trigger chord edited.
• Lossless editing: properties and comments it does not manage are preserved
  verbatim (including anything newer versions add), GUI state round-trips through a
  comment-based mmcfg metadata block, a rolling .gui-backup is written before every
  save, and the build-time embedded neo-mousekeys-ijkl default can be restored.

Engineering notes

• No dependencies: builds with the stock .NET Framework 4.x csc.exe via
  configurator/build.ps1; no NuGet packages, no project files, no build system.
  configuration/neo-mousekeys-ijkl.properties is embedded as the restore-default
  resource.
• DPI correct: PerMonitorV2 via the app manifest, all layout metrics designed for
  96 DPI and scaled to the current monitor, with WM_DPICHANGED relayout.
• Tested without a test framework: MouseMasterConfigurator.exe --self-test runs
  305 assertions headlessly (catalog coverage, chord parsing, conflict behavior,
  metadata round-trip, focus aliases, atomic restore, and a bilingual layout/text-fit
  pass over every fixed-size label). configurator/tests/FocusModeIntegrationTest.cs
  drives a real mousemaster.exe with injected keyboard events against a real
  foreground window (six checks: pass-through in idle, mode entry, pass-through exit,
  input swallowing, shortcut swallowing, clean restore).

Verification

• configurator/build.ps1 compiles cleanly and --self-test reports
  PASS: 305 assertions against this branch (i.e. with the current
  configuration/neo-mousekeys-ijkl.properties from main, which has evolved since
  v89 — the configurator's preserving parser handles the newer properties fine).
• On the fork, the same suite plus the focus-mode integration test (6 × OK + PASS)
  was run against the real upstream v89 mousemaster.exe, whose SHA-256 is verified
  against the upstream release in CI.

Out of scope (can follow if wanted)

The fork also has tag-triggered release automation
(.github/workflows/configurator-release.yml) that builds the configurator, runs the
self-test, verifies the upstream mousemaster.exe SHA-256, and publishes a bundle
(configurator + mousemaster + sample configs + SHA256SUMS.txt). It is intentionally
not part of this PR since release policy is the maintainer's call; happy to
contribute it separately if you are interested.

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