WaylandBackend: clipboard and primary selection support

[MithicSpirit]

This forwards the internal clipboard and primary selection to the host. However,
this does not yet add support for accessing the host's clipboard/selection
internally.

The source for both the clipboard and primary selection are recreated every
time, rather than being reused when they are still valid. Although this doesn't
seem to be necessary, it does appear to be the norm. In particular, the only
difference in behavior I could notice between the two methods is that clipboard
managers are not aware of the new copy when it is reused (this is at least the
case with wl-paste --watch under Sway).

This also fixes an issue introduced in d75b122, which broke forwarding selections
to the host altogether, including under the (previously-working) SDL backend.

[MithicSpirit]

It is of note that the primary selection support depends on the unstable primary-selection protocol. I don't care that much about that as compared to regular clipboard support, so I'm fine with dropping that part of the PR (the second commit) if preferred.

[misyltoad]

Oh! I didn't see this, thank you for doing this, I will look at merging it soon.

[MithicSpirit]

There seems to be a nontrivial merge conflict. While I think I figured out how to fix the conflict (just moving the SetSelection method to CWaylandConnector), I haven't been able to actually get it to work.

I believe that commit d75b122 is the issue (particularly the part where it commented out the call to SetSelection). Some basic testing on the SDL backend suggests copying to the host's clipboard is currently broken altogether.

[MithicSpirit]

I think I was able to fix this issue with e737059, but I'm not sure if this is implemented quite correctly (particularly, I'm not sure if the current connector is the correct way to do it). Either way, I was able to rebase on top of that (https://github.com/MithicSpirit/gamescope/tree/wayland-clipboard-2), and it seems to be working properly now. I can edit this PR to use that branch instead if desired.

[kisak-valve]

Hello @MithicSpirit, in general for any pull request, yes, you should update the branch you're proposing to be merged to be the best iteration you have. You should be able to force push the changes to your branch that this pull request is already pointing to and it'll show here.