Comment 5 for bug 1803156

We could have a CMake flag like DISABLE_LEGACY_CANVAS (which is, if not given, set automatically by the condition of HAVE_GTK3 (which implies Linux).

Then this can be use to mark out simple areas of code that can be culled later on. At least:

* Disable F9 hotkey
* Disable menu items
* On config read, convert canvas_type=0 to 1
* Any other ways to get a user into trouble with legacy + GTK3

And a stretch goal:

* any obvious areas that can be ifdeff'ed out without spending major effort to untangle (if it needs extensive refactoring, but doesn't actively cause harm in 5.1, perhaps it's a 6 thing)

I think on 5.1 release, quite a few distros will be using gtk3, so we don't want the bugs in legacy to be able to poke through too easily.