Add client indicators to highlight the focused client

Add client indicators to highlight the focused client

If the border around clients is very thin (e.g. 1 or 2 pixels), it can
be hard to tell at a glance which one is the focused client. This commit
adds client indicators, which are a bars at the top (in the bookshelf
layout) and the right (in the bookstack layout), which provide easily
visible cues about the currently active client.
The bars may further be used in the future to draw information about
the active and inactive clients.