Move internal accel map API there and update all users.
Also, add an internal function to create an accel path for
an action and parameter, and use it in gtkapplication.c and
gtkmodelmenuitem.c instead of duplicating that code.
Put this in a separate file and substantially refactor it.
Move handling of submenu creation into gtkmodelmenuitem where it
belongs.
Improve our handling of when to show separators or not.
GtkModelMenuItem, being based on GtkCheckMenuItem, was always reserving
space for a check/radio indicator. This ends up looking pretty strange
in a GtkMenuBar.
Don't reserve space for the indicator unless it's appropriate.
This GtkMenuItem subclass (and GActionObserver implementation) contains
all the knowledge necessary for converting a GMenuModel item description
into a GtkMenuItem.
Remove much of the code that used to do this from
gtkapplicationwindow.c.
