Search/Filter applies to Calendar

The search/filter selects in the activity list but doesn't
highlight the rides on the calendar.

This patch highlights the dates that have activities that
match with a red border and a red number.
Fixup search highlighting

.. on Linux/Windows since its slightly different.
.. also fixes a SEGV
Search highlighting on Calendar

.. hover color changes to differentiate.
Calendar highlighting tweaks

.. personal tastes, so should make configurable. But its nice to
make it the same as the a focus rectangle on Mac.