Fix mime type filter on background selection button

Summary:
Ported to explicit QFileDialog instance instead of static method partly
because I don't understand how to use the magic nameFilter syntax to set
mimes, partly to ease the eventual porting to QtQuick which can't have a
blockign dialog.

BUG: 402930

Test Plan:
Clicked on the button
Only image files were listed
Selecting one still worked

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18615