[bridge] QCoreAppWrapper is refactored

Sandbox thread becomes main thread from Qt point of view

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[bridge] removing qt code executed in the main thread

Next step in the way to prevent qt from saving main fuse thread id as
the main qt thread id. Still something to do

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>