Added rename and delete commands

[zesterer]

It's very common for me to accidentally create a file with the wrong name, be it due to a typo or just changing my mind about what the file should be called or where it should be. Previously, this meant:

1. Closing the file in Howl so that I don't accidentally end up re-saving it as its old name
2. Switching to my terminal / file explorer
3. Moving + renaming the file
4. Switching back to Howl, opening it via its new path

With the addition of this rename command, it's as simple as just typing out the new name/path I wish to give it, and Howl handles the rest.

For completeness, I've also added a delete command that deletes the underlying file and then disassociates the buffer with that file.

[zesterer]

Argh, crumbs, I've accidentally included a tonne of my personal changes here. Let me revise the history.

[zesterer]

As a point of interest, what's the maintenance status of Howl? I gather that there's been very little activity for a while. I'm interested in doing some low-level contribution. At least for the time being, it's probably going to remain my editor of choice.

[nilnor]

Hi! Maintenance status: I've been swamped with other things and hasn't much time to spend on Howl. I still use it myself for daily coding and all things, and as things have started to deteriorate with gtk-3 I'm "now" working on a Gtk-4 port. That's what I use now and will use going forward - it's in the gtk-4 branch but things are still a bit rough there with only the steinom theme ported.

As for the PR:

I'd skip the added methods for buffer and instead do it solely in the commands. That already has some checks for modification, etc., which require user interaction. Better to keep buffer as simple as possible, and use the available file property as is and keep the rest outside.