gtk: add command palette

[pluiedev]

GTK part of #5623

Depends on #5560 because writing custom GObject subclasses in Zig without a nice wrapper is like hitting your head with a sturdy piece of pickled mackerel

Currently still WIP: I think the only way for someone not to go insane with implementing complex widgets like this is to use either GTK Builder .ui files or to use Blueprint to generate UI files as a resource

[pluiedev]

Accidentally requested reviews from code owners: do not review this yet please!

[mitchellh]

Surprisingly simple! I didn't heavily review the real meat of the action palette GTK stuff, just a cursory look. But I looked at the rest.

I think we should try to find a way to limit scope on this and split it up to make it a bit easier. We already have the gobject PR that's separate which is nice. I think I want to do the command palette core work in a separate PR that you can then try to rebase this on top of.

I also think introducing Blueprints (something I've always been supportive of!) as part of this makes it slightly more complicated too. Can we introduce blueprints for something simpler (say... the unsafe paste dialog or something) and get that in a separate PR so we can get all the machinery we need to work with blueprints within the project separate from a massive feature?

And finally, this should all of course wait for 1.2 😄