Server-side decorations support

[keithbowes]

A while back, @icedman created PR #24, but never addressed the problems. If someone wants to pick this up and make it possible to merge, that would be great. The problems with the PR upon its creation:

1. It doesn't pass the build tests.
2. It uses KDE's obsolete server_decoration protocol. It'll have to use the official xdg-decoration protocol.
3. Some of the source files are huge (so huge that GitHub won't even show them). If that's necessary, that's OK, but I have a feeling it's not. In particular, rendering XPM files shouldn't be something to do. XPM files can easily be converted to SVG and windowing system-agnostic libraries can be used for them (instead of adding a bunch of code to Waybox to handle a legacy format).
4. It adds a lot of new themes. I don't think these should be added to the Waybox repo. Maybe the default theme should, but largely, I'd rather a different repo for themes (ideally, one arranged so that someone could just use git clone in $XDG_DATA_HOME/themes (or $prefix/themes)).

Additional problems now:

1. There are merge conflicts.
2. It relies on the wlroots gles2 renderer (optional since wlroots 0.14.0), so it's not renderer agnostic. I suppose we could change meson.build to build it only when wlroots has a gles2 renderer, but I'd prefer something that could be used with any renderer.
3. It forces server-side decorations. Client-side decorations should be used if the client requests them. I think we should use Openbox's <decor> element (only making it global, instead of only possible in <application>), with the Openbox values yes and no, as well as client (always use client-side decorations, even if server-side decorations were requested) and server (always use server-side decorations, even if client-side decorations were requested). However, I'd be implementing that option and it would only be the job of whoever takes this up to make sure that client-side decorations are used if they're requested (it's easy to detect: wlr_xdg_toplevel_decoration_v1.requested_mode).