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.

[dagargo]

I'm actively working on this right now. I've pushed all the changes to the feat-gtk4 branch. For anyone reading this, I've recreated the branch and started again.

Right now, many things have been disabled and there are many deprecation warnings but it compiles and runs. See the commits for a more detailed explanation of what is working and what is not.

[dagargo]

I've pushed more commits and many things are working now as they did.
The only remaining things are the browsers popups and the drag and drop code.

Once this is done, I'll squash everything into one single big commit. Then, I'll continue with the migration of the deprecated widgets into different commits. These should not break things.

As the deprecation warnings are gonna be kept, long term maintenance tasks should be distributed more evenly.

[dagargo]

Browser popups are working again. The behaviour is weird sometimes when clicking on the items but since the code will be rewritten to replace GtkTreeView with GtkColumnView I don't think this is really important now.