keytests: Implemented use of additional shortcuts in keytest.py

keytests: Implemented use of additional shortcuts in keytest.py

Now, before starting a test with 'TestBegin' in any *-in.txt file
we can create ne shortcuts used by this test.
Defining a shortcut:
	UseShortcut "<shortcut>" "<function>"
Actually adding the new shortcuts to be used by the next lyx-session:
	PrepareShortcuts

Use example is found in findadv-combined-in.txt
The UseShortcut-entries will be traslated into '\bind "<shortcut>" "<function>"'
and with the PrepareShortcuts-entry added to the active use file 'user.bind'