Calendar: Implement saving, loading, and displaying of calendars

The user can now save, load, and view calendars. A calendar is made up
of an array of events which are saved in a JSON file. In the future we
should implement the iCalendar standard instead of using a custom
format.

Base/etc/FileIconProvider.ini

@@ -30,6 +30,7 @@ ruby=*.rb
 shell=*.sh,*.bash,*.zsh
 sound=*.wav,*.flac,*.mp3,*.qoa
 spreadsheet=*.sheets,*.csv
+calendar=*.cal
 text=*.txt
 truetype=*.ttf
 pixelpaint=*.pp