Prepare for GTK4 porting

The changes from this merge request should ease the transition to GTK4, are all working with the GTK3 version as far as tested, so proposing to merge to be able to work on master with GTK4 porting and have testing for the changes done here (mostly refactoring of the function and variable popover). Summarizing work done:

• refactored function and variable popover to have a common ancestor handling most of the gtk-specific work
• changed the way space character is removed from function and variable name to use changed signal instead of key_press event to avoid the need for key controller
• use a Glib ListStore and bind that as a model to the function/variable popover instead of working directly with ListBoxRows
• use EventController key on the application window instead of catching the key_press event