Custom context menus are overwritten

[markfinal]

This is a regression from the fix for #46

There are three places where a custom context menu is shown:

Local cache

• adding a new environment variable, with a context menu with standard Conan variables
• adding a new remote URL, with a context menu with previous URLs used
• installing a new configuration, with a context menu with previous paths to configurations used

What happens is that the procedural submenu now overwrites the parent menu.

The statement in question that was added in the previous fix is

    def contextMenuEvent(self, event: QtGui.QContextMenuEvent) -> None:
        menu = self.createStandardContextMenu()
        if self._custom_menu:
            menu.addSeparator()
            self._custom_menu.setParent(menu) <-- this
            menu.addMenu(self._custom_menu)
        menu.exec_(event.globalPos())

which if removed does resolve the problem, but this was added to resolve warnings on Linux.