build: Remove desktop generation duplicated commands

Every plugin generate an autostart desktop file, except `common`
and `dummy` that are special cases.

This desktop file generation is duplicated among the different
plugin's build files.

This code has been moved to the build file in the `plugins`
directory, avoiding any code duplication.

The downside of this is that meson is not able to generate files
in a different directory, so all the files will be generated in
the `plugin` directory, outside of the each plugin's directory.

This is something that it's worked on in a different feature and
it might be merged in the future.

See https://github.com/mesonbuild/meson/pull/2617

https://bugzilla.gnome.org/show_bug.cgi?id=793087