Migrate to GTK4

[dagargo]

This is the plan.

• Rename UI file
• Replace custom dialogs with windows
• Replace GtkMenus with GtkPopoverMenus
• Change dependencies and compilation options in configure.ac and Makefiles to GTK4
• Migrate ui files to Cambalache
• Replace GtkComboBox with GtkDropDown (GTK4 4.8)
• Create GObjects for the GtkColumnView models
• Replace GtkTreeView (GTK4 4.12)

All this is gonna be implemented in the feat-gtk4 branch.

[dagargo]

All this is gonna be implemented in the feat-gtk4 branch.

Perhaps that was not a great idea in the sense that many of the changes above are GTK3 compatible and can be done directly onto the master branch. Merging this would simplify the workflow because migrating to GTK4 is a lot of work and it's gonna be easier and smoother to work onto master as much as possible.

[dagargo]

As I mentioned, all these changes are gonna be made directly onto the master branch progressively until the code is as much ready as possible for GTK 4.0.

After this, fixing deprecated symbols will be handled for different releases until everything is resolved.