Make accidentally activating the link navigation more difficult

Make accidentally activating the link navigation more difficult

Only checking Ctrl and link availability on mouse release was triggering
the navigation too easily.

Now you have to press and release the mouse on the same link, having
Ctrl pressed all the time. In addition, when changing the text selection
the link is also cleared.

Task-number: QTCREATORBUG-134
Reviewed-by: con
(cherry picked from commit 2c115e934fb5700e42f64491af92b3facfb2cbf0)