qmljs: better defaults, more uniform handling of paths

Uniform handling of the different qml dialects.

Now paths for a given document prefer the project of that document.

For Qt the following sequence is used:
- Qt of the project of the document
- Qt of the active target
- Qt used to run creator

Currently all paths of the open projects are still merged, but that
can be changed.

Change-Id: Id302c13c893b66fbfe24e301602fe69de152eed4
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>