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