Add a plugin to move through layers, changing visibility and group pos

This plugin adds two actions

"LayerGroupSwitcher/next
LayerGroupSwitcher/previous

without default shortcuts for now. The actions activate the layer above
or below the current layer with two twists:

* the current active layer is made invisible, and the activated layer
is made visible

* if you're in a group layer, the group is made invisible, the next
or previous group is made visible and if there is a layer inside the
newly activated group at that index is activated.