i3bar: Verbose workspace title

Modify i3 IPC output for get_workspaces, when printing workspace name,
append the currently focused window name too. So that i3bar could
display the current focused window title.

Only new window and workspace events are triggered so that when swithing
to another window in current workspace won't get i3bar updated.  Also
noticed that urxvt's window name is initially urxvt rather than $PS1
assigned in your .bashrc.

Need a method to notify i3bar on these cases.
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)
IPC: limit output length of title

Only output first 30 glyphs of title.
ipc: skip container (non-window and non-leaf)
ipc: shorter ws title
add screenshot.
Merge branch 'master' of https://github.com/famz/i3