Do not allow that panels are dragged out of the main window

This feature might have been useful in some cases, but it's most likely
not useful enough to justify that we accept the bugs that it causes.
Even though some of these bugs are not Dolphin's fault at all, they
cause serious problems for users in some cases.

I'm pushing this commit to master only because I prefer to not push such
behavior changes to 4.11 at this point of the release cycle.




FIXED-IN: 4.12.0
REVIEW: 111692