[test/xdgtest] Create/destroy popup on click

Summary:
This allows a dev to move the window before creating the popup which
is extremely useful in testing constraints. Also makes it very easy to
mod this code into a grabbing popup for other tests.

This patch also improve the painted surfaces to show the anchor rect
around where we place the popup which is easier for visual debugging.

No library code changes

Test Plan:
Ran the test
KWin doesn't position the popup according to all constraints
Soon will

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D16294