Improve positioning of menu

[Matthias Schoettle]

The menu in DisplayAspectScene is set according to its width and the screen width, with the position anchor of the menu being the top left. This requires that whenever the menu is changed (adding or deleting buttons) it has to be repositioned as the width of the menu probably has changed (which lead to repetitive code).

Maybe it can be improved by using the position anchor top right, such that the position can be set once to the screen width.

[Matthias Schoettle]

Resolved with commit 07ed2eb.

UPPER_RIGHT had to be added to MT4js PositionAnchor in MTRetangle. This now allows to set the anchor to the top right corner of a component and set the position accordingly.