Prepare plugin for PyQt6 and QGIS 4

[merydian]

Will make the plugin usable under QGIS 4. Also includes some refactoring for structure and readability.

Closes #23.

[falkmielke]

Updating this plugin for QGIS 4 would be much appreciated.
Much appreciated that this PR goes in several steps and commits.
I attempted using the adjusted code from this PR to locally zip and install the newer version.
However, out of the box, some ui files seemed to be missing:

An errCouldn't load plugin 'changeDataSource' due to an error when calling its classFactory() method 

PyQt6.uic.exceptions.UIFileException: /QGIS/QGIS4/profiles/default/python/plugins/changeDataSource/ui/changeDataSource_dialog_base.ui: invalid Qt Designer file: [Errno 2] No such file or directory: '/QGIS/QGIS4/profiles/default/python/plugins/changeDataSource/ui/changeDataSource_dialog_base.ui' 
Traceback (most recent call last):
  File "/usr/lib/python3.14/site-packages/PyQt6/uic/ui_file.py", line 35, in __init__
    document = ElementTree.parse(ui_file)
  File "/usr/lib/python3.14/xml/etree/ElementTree.py", line 1214, in parse
    tree.parse(source, parser)
    ~~~~~~~~~~^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.14/xml/etree/ElementTree.py", line 566, in parse
    source = open(source, "rb")
FileNotFoundError: [Errno 2] No such file or directory: '/QGIS/QGIS4/profiles/default/python/plugins/changeDataSource/ui/changeDataSource_dialog_base.ui'

Python version: 3.14.5 (main, May 10 2026, 18:26:20) [GCC 16.1.1 20260430] 
QGIS version: 4.0.2-Norrköping Norrköping, exported 

This might just be an issue of me putting the files in the wrong place: I have not developed QGIS plugins ever and was just hoping that a zip of the source folder would install.

Yet I think that the issue is only in the "refactoring" part of the PR.
In my opinion the "refactoring" and the update should have been submitted separately.
I just went back to the first commit in this PR with git checkout e432926 and the plugin works fine: thus, up until that commit, this would be an alternative to #25 .

[nicpitsch]

+1