Add LFUN_SERVER_GET_STATISTICS command

This function can be used with the LyX server to obtain the word, character or character+space count in the current document or selection.
Cmake build: No need to recreate lyx.pot on every make call.

We can check if imput data for 'gettext' command has changed from previous call.
If not, there is no need to recreate lyx.pot, which we only differ in
	"POT-Creation-Date: ...."
Merge remote-tracking branch 'github/build-bot-master' into bleeding-edge-master
2014.10.23-15.00