app: Fix emit_event()

Now an event either has a source or doesn't, i.e. the triggers are found
under "events/<EVENT>" or "<SOURCE>/events/<EVENT>"

There's always only one category. If treeviews are to emit events, they'll
simply call emit_event() twice, once with "tree_views/<NAME>" as source,
and once with "defaults/<MODE>s" as source.
app: Add emit_info(), helper for event "info"

Event "info" will be used to send messages on some actions, so user can
have feedback. E.g. when nodes are copied to a register, etc
provider-register: Emit "info" when setting/adding/dropping register
treeview: Emit "info" when loaded from/saved to a file
app: statusbar: Add ":app" as source

Supports showing the version number, and the message & details from the
last event "info"
provider-register: Fix getting clipboard context after a drop

It is possible that we dropped register '+' and yet still own the clipboard,
and are asked for its content. Make sure this is properly handled.
list: Fix status not always updated on empty location

Since 559bc45b statuses weren't checked/updated when changing location
to an empty one.