app: added Add configuration action

GProxies.Row: added public API
GProxies.Window: added handling of the action
GProxies.Window: keeps a reference to the active row
window: added hook point to row_activated signal
row: improved behavior, added more public API

Fow now, insensitive labels will denote a non-ready state while
edititing the details of a particular configuration.
On the other hand, user can only delete non-selected rows.
window: modified default size
listbox: added separator between rows
docs: added Roadmap
row: added proper Save button indicator
app: renamed appmenu id
app: added menu for default plugin toggling