main/postmarketos-hidden-desktop-entries: modernize

Use replaces="" to overwrite files in /usr/share/applications, instead of creating files for /etc/skel/.local/share/applications. This way, we can add new overrides when upgrading the package, not only initially when creating the user's home dir from /etc/skel.

Remove v20.05 related comment, and "firefox-safe.desktop" override, which is now "org.mozilla.firefox-safe.desktop".

Add org.gnome.Glade.desktop from glade, which gets pulled in by libhandy1-dev (e.g. when installing makedepends for phosh). (Users who really want to start glade on postmarketOS can either do it with the command line, or uninstall postmarketos-hidden-desktop-entries, or replace the file in /etc/ with the original .desktop entry. For everyone else who just installed glade as makedepend, it removes one awkward desktop launcher.)

I looked into modernizing this package to hide launchers that appeared for GTK4 (issue 1029). However, this should be fixed in Alpine's gtk package: https://gitlab.alpinelinux.org/alpine/aports/-/merge_requests/20116

Related: #1029 (closed)