When pasting clipboard content from Godot to other applications,
multiple SelectionRequest events are sent to Godot in order to access
the data. It could take a long time before the data is ready for the
other app because events were processed one by one on the main thread,
especially when Godot is unfocused and runs at low frequency.
With this change, SelectionRequest events are directly handled on the
separate event polling thread to minimize this delay.
This change also replaces clipboard_get() calls in SelectionRequest with
a direct access to internal_clipboard, since in this case we know Godot
is the owner of the clipboard content and it's not necessary to query
the x server for it.
5a0376f969
