GTK3 clipboard support

[totaam]

Issue migrated from trac ticket # 1589

component: client | priority: major | resolution: fixed

2017-07-20 14:20:50: antoine created the issue

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This is going to be fun, not.

Split from #640. See also #1568 for generic python 3 issues.

Might be a good time to bypass gtk completely: #812.

[totaam]

2017-07-20 17:43:39: antoine changed status from new to assigned

[totaam]

2017-07-20 17:43:39: antoine edited the issue description

[totaam]

2017-09-17 13:04:37: antoine changed status from assigned to closed

[totaam]

2017-09-17 13:04:37: antoine set resolution to fixed

[totaam]

2017-09-17 13:04:37: antoine commented

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Done in r16892 with one important caveat: we can't update the clipboard contents on the SelectionData object we're given with Gtk.SelectionData.set (no idea why), so we update the clipboard and claim ownership of it. (we own the token)
This only works for text contents..