Bump version number
Add version checking macros

So you can do things like GCR_CHECK_VERSION(x,y,z)
eggimagemenuitem: Update comments about for GtkImageMenuItem

Reasoning here: https://docs.google.com/document/d/1KCVPoYQBqMbDP11tHPpjW6uaEHrvLUmcDPqKAppCY8o/pub

Although it would be nice to port to GtkMenuButton, doing so would be
breaking ABI, and can't be done until 4.x (likely in sync with GTK+ 4.x)

We still want to continue using icons in our dropdown import button.
So the 'egging' of GtkImageMenuitem still stands.