editor: implement Cut,Copy,Paste

editor: implement Cut,Copy,Paste

weston-editor is the only stock client spawning the virtual
keyboard ; which means it may be the only client able to
obtain some special characters (depending on the user's
keyboard layout).

If we implement Cut, Copy and Paste, the user has now a way
to copy such characters to other useful clients (such as
weston-terminal). Plus, it demonstrates text data exchange
between two clients of different nature.

Functionality is implemented in a right-click menu and the
Ctrl+Shift+X/C/V bindings, just as in weston-terminal.

Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Tested-by: Bryce Harrington <bryce@osg.samsung.com>