Extend filtering: negations, calendar-specific regular expressions

[hofheinz]

Allow to filter events based on

• negations (e.g., "!Birthday" filters everything out except birthdays),
• calendar-specific regular expressions (e.g., "RE:CAL=cal1;TITLE=.[wW]ork." filters out all events in calendar "cal1" that contain the word "work" or "Work").

[hofheinz]

This is a little hackish, for a number of reasons:

• This passes the calendar of the event as a String argument to KeywordsFilter.matched(). It would possibly be cleaner to pass an object with more information.
• It would be nicer to write a more complete parser (with OR, AND, etc.).
• Currently REs are rather expensive, some kind of cache for compiled REs might be nice.

I'd be happy to work on any of these things, but wanted to first of all put this kind of improvement up for discussion. (Maybe it's not what you feel is useful, feel free to reject of course.)