Fix paste of selection to (unfocused) external applications

Fix paste of selection to (unfocused) external applications

With both Qt4 and Qt5, when using a click-to-focus policy, the first
attempt to paste a selection by middle mouse in an external application
which has no focus may fail. It is not clear why this succeeds for some
applications and fails for others, but refreshing the timestamp of the
selection request cures the issue. The cmake part is by Kornel.

See also this thread:
http://thread.gmane.org/gmane.editors.lyx.devel/162491