Clipboard updates

[matt335672]

Fixes #2763

This PR contains three commits

1. The first of these adds more checking to a Format Data Response PDU. In particular, if the client responds with CB_RESPONSE_FAIL for any requested format, the X11 client is informed that the requested selection is not available, rather than the message being ignored. Additionally, pasted text is now given its own function, and more error checking is added.
2. The second of these only advertises text to the X11 clients when a text format (or a file list) is advertised by the Windows client.
3. The third of these is a tidy-up. There's a lot more that can be done in this area, but I've stuck with the minimum so that a backport to v0.9.x is feasible.

With these commits, when a picture is selected on the Windows client, the following can be seen using xclip:-

$ xclip -o -selection clipboard -t TARGETS
TARGETS
TIMESTAMP
MULTIPLE
image/bmp
$ xclip -o -selection clipboard -t STRING
Error: target STRING not available

[metalefty]

Thanks for the fix, I'll test. I think backport to v0.9 is necessary.

[matt335672]

The patches I've added cherry-pick nicely on to v0.9, which is one reason I didn't make more changes in this area! I'll do some more tidy-ups on devel only.

Since the cherry-pick was so trivial, I've added #2767 against v0.9. Test whichever one works best for you and I'll apply any fixes to both.

[metalefty]

LGTM.

% xclip -o -selection clipboard -t TARGETS
TARGETS
TIMESTAMP
MULTIPLE
image/bmp

% xclip -o -selection clipboard -t STRING
xclip: Error: window id 0x1c00001 cannot convert CLIPBOARD selection to target 'STRING'