GTK4 Migration Implementation

[devin-ai-integration]

This PR implements the GTK4 migration as outlined in issue solvespace#853.

Changes

• Created new guigtk4.cpp file based on guigtk.cpp
• Updated CMakeLists.txt to conditionally include guigtk4.cpp when GTK4 is available
• Updated menu implementation to use Gio::Menu and Gio::SimpleAction
• Updated event handling to use event controllers (GestureClick, EventControllerMotion, etc.)
• Updated window management code for GTK4 compatibility
• Updated file dialogs to use GTK4 APIs
• Updated application initialization to use Gtk::Application instead of Gtk::Main

Notes

• The code has been built successfully with GTK3, but couldn't be tested with GTK4 libraries as they weren't available in the standard Ubuntu repository
• The changes follow the migration checklist from issue Prep for GTK4 solvespace/solvespace#853
• This implementation should be tested by someone with GTK4 libraries available

Link to Devin run: https://app.devin.ai/sessions/82a2b848bf614a0caaa05d68083e1438
Requested by: Erkin Alp Güney

[devin-ai-integration]

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