Split up the glade file

This UI file splits siril3.glade into numerous .ui files in a new subdirectory src/gui/uifiles

Each GtkDialog, GtkColorChooserDialog and GtkApplicationDialog gets its own UI file; there is also a single UI file for the file filters and one for the menus. (The menus could be further split down, but for now I think this is enough.)

There is no change in functionality with this MR, the idea is to get it merged as quickly as possible to minimize the disruption resulting from replacing siril3.glade.

Once it is merged, a second MR will add compilation of the UI files into a GtkResource. This will avoid the need to leave lots of .ui files lying around on users' filesystems. Subsequently, developers may wish to consider migrating individual .ui files out of the startup UI loading function so that they "lazy load" when the dialog is first used. This is not trivial, however, as there are currently a few functions that rely on the entire UI being loaded from startup (dark / light icon theming, the function added to ensure on_close callbacks are added to some dialogs in the correct order, possibly some others).