Performance problem

after adding PR (#232), I noticed a few bugs
1. when scrolling through months, if you go far enough (for example, in 1999), you notice increased memory consumption


this is because the array of months is not cleared. after closing the widget, everything remains in memory.

2. when switching to a new month, a lot of unnecessary actions are performed.


The calendar model changes 4 times (per iteration), each time it completely redraws everything related to events.

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