switcher: Remove some unused variables
calendar: Stop computing week number ourselves

Correctly computing the ISO week number is tricky and we already
have code in the platform to do it, so just refer its computation
to GDateTime rather than doing it ourselves.

https://bugzilla.gnome.org/show_bug.cgi?id=736722
build: Add org.gtk.Application.xml to distfiles
Bump version to 3.15.2

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