Exclude . and .. when looking for plugins

Avoid current and top level directories.
Expose filter on preflets proxy model as a property

Proxy model is simply filtered by setting the filter property.
Remove const qualifier

This will let us create preflet components from the item() method.
Create Background preflet component the first time it's needed

Component is not created as soon as the plugin is loaded anymore, we
create it the first time it's needed.
Qt5Xdg and Qt5AccountsService are not hard dependencies anymore

If such modules are not available we won't build the plugins that
require them.
Use GNUInstallDirectory instead of HawaiiInstallDirectory

This will avoid remaining Vibe dependency.
Install preflets under Qt plugins directory

So we won't need to set QT_PLUGIN_PATH environment variable.