rc.lua: Fix titlebar button events (FS#1116)

Due to some layout changes, the client title widget is no longer assigned all
the remaining space. To work around this, we wrap the widget in a flex layout
which means that it asks for all the available space. This way, moving via the
titlebar works again.

Additionally, these button events are now also assigned to the client's icon
widget. No idea why this wasn't done before...

Signed-off-by: Uli Schlachter <psychon@znc.in>