Allow mdi subwindow handle context menu event.

ViTables handles context menu event for mdi subwindows. Thus there is no obvious way to write a plugin that creates custom data view and requires left mouse button events (for example data plot). To solve this problem I changed `eventFilter()` in `vtgui`. It was intercepting and handling context menu events inside mdi area. I added additional check: if there is an active subwindow object, it has the field `is_context_menu_custom` and it is set to true then the event is passed to the object. Thus context menu works as before for the build in stuff but this event can be handled by a plugin if it creates an mdi subwindow.