Guard dialog in CPN nets enfoces paranthesis

[Jiri Srba]

If you edit transition guards in colored nets and press several times
AND or OR, you get the following:

(<*> and <*>) and <*>

and the dialog and parser enforces the useless paranthesis. If you e.g. do manual
edit and write just

<*> and <*> and <*>

where <*> is replaced with some concrete expressions, it refuses to parse it due to
the missing paranthesis. This should be fixed, so that it behaves as the query dialog
(where the paranthesis are option but not necessary).