Change context menu handling of SvTreeListBox

Status quo

When using the Right-Mouse-Button to rise up the context menu, the context always applies to the last selected entry/entries. Thus when the selection is not actual visible, it is not clear to which entry/entries the menu belongs to. In addition, the menu may be opened far away from the selection which it refers to.

When rising up the context menu by keyboard (shift-F10), the menu is placed at the actual mouse position and not at the position where the selected entry is/entries are located.

How it will be

Handling right mouse button click

Handling context menu request by keyboard