make the client shutdown timeout configurable

sometimes the timeout for applications to shutdown is too short, e.g.
when running on old hardware, having lots of applications open,
browsers with many tabs, etc.
This patch adds two config entries with which you can define the
timeout before ksmserver kills the apps.
In the [General] section of ksmserverrc you can define:
legacySaveTimeoutSecs (default = 4 seconds)
clientShutdownTimeoutSecs (default = 15 seconds; was 10 before)

BUG: 248459
BUG: 273326
FIXED-IN: 4.9.1
REVIEW: 106090