The all/current project searches use metacalls on the UI thread to work
around thread-unsafety of the Project::allFiles method, so we cannot jus
t block the UI thread to wait for the old search to cancel.
On the other hand, many of the ILocatorFilter::matchesFor implementation
s do not work when they are run in multiple threads simultaneously (most
implementations access unguarded member variables, help index filter tend
s to crash), so we _do_ have to wait for the search to cancel before sta
rting a new thread.
Broke with 05c26767 and 6fc39f0c


00c5673ae80d03d115bf3739

Change-Id: I93c607c879e8ba6ef60f94719812edb3df43a154
Task-number: QTCREATORBUG-12875
Task-number: QTCREATORBUG-12592
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>