Add option to bind mouse thumb buttons for mouse-centered rotation

[jwt27]

New idea for the ROTATE_AROUND_MOUSE option. I really do think it's an improvement to have this on the mouse thumb buttons, so I propose adding an option value to pseudo-hardcode these buttons, without changing any default keybinds. The probably-confusing ONLY_CTRL/NOT_CTRL options are removed (although, I'm not entirely convinced I made it any less confusing overall).

[PieterVdc]

for keybind preferences user should just rebind, imo this is just clutter added to the cfg

[jwt27]

Yes, I completely agree. But this is not just a rebind, I want to introduce this new feature (mouse-centered rotate) specifically for the thumb buttons only. This is consistent with mouse-centered zoom on scroll wheel, both are controlled by the same hand. However, scroll wheel zooming is hardcoded, which I don't want to do for this. So an extra config option is then the best choice, if changing the default keybinds is not possible.

Surely though, it must be possible to change some default options, in some cases. This shouldn't affect anyone's existing settings. I would've thought the Delete/PageDown keys, being entirely redundant and presumably little-used, wouldn't really be missed.

[jwt27]

I changed it again. Now ROTATE_AROUND_MOUSE is just a binary option to bind the thumb buttons, on or off. The FOLLOW option is also removed, I don't think it has any practical use.