qml: minor optimization
- Set root type
Itemrather thanRectangle. See docs:
Applications with a monochrome background should set it using QQuickWindow::setColor() rather than using a top-level Rectangle item. QQuickWindow::setColor() will be used in a call to glClear(), which is potentially faster.
- Constraint miniplayer background effect source to
stackView, so that updates on the playlist or other irrelevant areas do not cause the effect to be updated (It is assumed playlist is never part of miniplayer background).