i3bar: Repair tray client handling

Show/hide tray clients asynchronously on MapNotify and UnmapNotify respectively.

Remove tray clients on DestroyNotify as per the EXembed protocol
specification (see:
http://standards.freedesktop.org/xembed-spec/xembed-spec-latest.html)

Add sanity checks and safety measures to guard against sending messages
to tray clients that could result in BadWindow errors by tray clients
(but that issue is not entirely resolved here).

Adds support for QSystemTrayIcon class, qt-base version 5.3 (qt-base
version 5.1 is still unsupported)

fixes #1110.
disable mouse on bar
disable scrolling on window decoration