screencast: Use sessionUpdated as our method name

This is what we consistently use everywhere else.
components: Remove the built-in recorder keybinding and component

The keybinding is now part of gnome-settings-daemon
Add a screencast indicator for when we're recording

This will replace the indicator painted on the stage right now.

This unfortunately does not work for the recorder triggered by the
keybinding -- we'll simply replace the in-shell code with a keybinding
powered by gnome-settings-daemon.