[PowerTimerEntry.py] add summary-description for Timer-Entry, works now ...

...also on/for default TimerEntry

TODO: translated Title is needed(DE) for that at the moment. This is the entry:
self.setTitle(_("PowerManager entry"))
it's  in the enigma2.pot at line 14902.

Example for skinners:

	<screen name="TimerEntry_summary" position="0,0" size="256,64">
		<widget source="parent.Title" render="Label" position="120,0" size="136,40" font="FdLcD;17" halign="center" valgin="bottom" />
		<widget source="parent.summary_description" render="Label" position="0,42" size="256,22" zPosition="1" font="FdLcD;20" halign="center" valign="center" />		
	</screen>