qml: keep context menu button visible when menu is shown

for some reason, it always bothered me that the context menu button vanishes when the context menu is shown in the table view, here is a fix for this.

Implementation - instead of a simple signal used currently to request a context menu in TableView, an object with certain properties like context menu visibility and menu request ID is required by TableView to help us determine context menu state and if TableView requested it. This is used to toggle the visibility of the context menu button. I've also introduced a common base class in qml_menu_wrapper class to help with this.

(review request @chub)