krunner: polish placement
Tell the parent Dialog class what the location is so it can configure
the dialog accordingly.
Install the desktop file where it can be found by kwin to know about
While at it, make sure we don't leak if we return early.
Test Plan: Now I get the right animation
Reviewers: Plasma, cblack, davidedmundson
Reviewed By: Plasma, cblack, davidedmundson
Subscribers: davidedmundson, plasma-devel
Differential Revision: https://phabricator.kde.org/D28783