`ext_messages` support

[Lyude]

It's very unpleasant not being able to select text from messages in neovim-gtk, and we could fix this and much more. Ideally, I'd like to make it so that we can do full GUI dialog prompts, and for displaying the message buffer: actually use a proper GtkTextView widget.

Since ext_messages also implies ext_cmdline according to nvim's documentation, this is enough motivation to actually improve our commandline widget as well. Ideally I'd like us to both have a popover mode for it like we currently have, but also provide a "transparent" mode where we do our best to draw the command line with our own widget while taking care to ensure it's visually identical to not having ext_cmdline enabled. (And hopefully with that, we could turn ext_cmdline on by default where it's supported without breaking anyone's setups…)