Today check in CSSCalendar slows down application

[ShadowKyogre]

Bug is in current version of application and in wip branch. Steps to prove this:

1. Open chronoslnxlib/csscalendar.py
2. Comment out the highlighted lines in the link https://github.com/ShadowKyogre/ChronosLNX/blob/wip/chronoslnxlib/csscalendar.py#L14-L20
3. Run ChronosLNX (via the wrapper *.py)
4. Try resizing the window in any direction

Expected result: Window resizes without delay
Actual result: Window lags while resizing

Proposed solution: Store whether a date is "today" so the today checks aren't done on every redraw.