The bug report showed a situation where contextDisposed is called from a
non-UI thread. Calls on Display#removeFilter must make sure to run on
the UI thread. Assuring this by running syncExec() on the given Display.

MaximizableChildrenTag: Changed implementation of UISynchronize to the
one from E4Application#createE4Workbench.

Change-Id: Ia2bf8ab1702665601659b14d710b2215e80d77d4
Signed-off-by: Karsten Thoms <karsten.thoms@karakun.com>