QMenuProxy & Plasma::Types::PopupPlacement: Reverse direction for RTL

Function naming is taken directly from QStyle where visualRect() is used
to flip rectangles within components' bounding boxes automatically if needed.

The QQmlProperty trick is taken from some internal Qt test. It relies on
assumption that QtQuick module is imported without an alias, which is
the case in 99% of pragmatic QML code, with exceptions being Qt tests,
Qt3D and JavaScript modules with QML/JS engine extension of importing
QML modules. Overall, in ~/kde/src:

  Searching 324578 files for "import QtQuick (\d\.\d{1,2} )?as" (regex, case sensitive)
  ...
  114 matches across 109 files