Add persistent client properties

`client.property.persist` can be used to make properties persistent
across restarts. This works by backing them up via X properties (using
the `awful.client.property.` prefix).

`client.property.persist(c, "floating", "boolean")` is used to make the
floating property persistent by default.

Based on a patch from Uli, source: https://gist.github.com/psychon/10320743

doc, only set current prop in 'persist'

Fix xprop/prop mixup in 'persist'

Only call set for non-nil values
Call AllowEvents after grabbed events on a drawin

When we get an event due to a previous GrabButtons call, we have to continue
normal event processing again, because the server froze the input device for us.
Without this, everything appears to freeze.

Signed-off-by: Uli Schlachter <psychon@znc.in>
Merge remote-tracking branch 'blueyed/remember-layout-across-restarts'