krunner: polish placement
Summary:
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
X-KDE-Wayland-Interfaces=org_kde_plasma_window_management.
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
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D28783