calendarProvider: do not use "hit-and-miss" method when getting event counts for the month. Look for files matching a specific format instead.
This speeds up the monthly calendar view in a very noticeable way. Now the time a month takes to load is proportional to the amount of days with events on it. A month with few events will load very quickly even if the database is full of events, unlike what happened before.
calendarProvider: noticeably speed up event search using the method applied in the changes of the previous commit.