Windows GUI: Code clean-up, organization and fixes

Remove unused donate buttons and related code
- As mentioned in https://github.com/MediaArea/MediaInfo/issues/891#issuecomment-2200575320
Organize GUI_Main.cpp
- Move functions to the appropriate sections and improve some comments for easier finding and understanding in the future
Upgrade open file dialog
- Use the same dialog type for opening files as the one for opening folders
- Should not have visible difference since old dialog is auto upgraded by VCL at runtime to new one when running on Windows Vista+
- This change is for consistency with folder open dialog and maybe make it more efficient by dropping support of old dialog
- The new dialogs work on Windows Vista+
Partial fix for 2nd issue of #891
- The GUI is now properly displayed following user preferences when Preferences window is closed
- The issues that happen when Preferences->Advanced is opened remains
Fix the file selector button in Export window is too big at high-DPI
- Similar to recent changes for buttons in Main and Preferences windows

MediaArea / MediaInfo