Windows GUI: Code clean-up, organization and fixes

[cjee21]

• Remove unused donate buttons and related code
  • As mentioned in 24.06 - Open Preferences and click on "Adavanced" -> GUI buttons appear #891 (comment)
• 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 24.06 - Open Preferences and click on "Adavanced" -> GUI buttons appear #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

[JeromeMartinez]

Thank you for the clean up!