ipc: Add title for get_workspaces

Added title field in json output to hint i3bar for workspace button
rendering. Defaulted to the same with "name" when new ws created. If a
window in ws is focused, window name is appended to workspace title.
IPC: Add IPC notifications.

Added notifications for below events:
 * Window name change. (window notification)
 * Window focus. (window notification)
 * Window destroy. (workspace notification)