Show a loading spinner if the local package takes some time to read

This can happen if the package is very large.
Avoid bumping the GTK version just for one flag
trivial: Include git.mk in tests
Add a file handler to allow Software to install local packages
Allow multiple plugins to set the name, summary and description

Only the plugin with the highest 'data quality' will win. This allows us to
nuke some more metadata and also take some internal assumptions out of the code.