Add mimeapps.list, so links clicked in postmarketos-welcome and other
applications properly open in firefox. Without this patch, they just
don't open at all.
Bump the pkgver instead of the pkgrel, so the APKBUILD is in sync again
on both master and v20.05 (right now, the pkgrel on v20.05 is ahead by
one, otherwise the file is the same).
Related: https://help.gnome.org/admin/system-admin-guide/stable/mime-types-application.html.en