Menuitem for 'Display format NUnit': simplify 'show namespace' submenu

[rowo360]

This PR fixes #1204 by replacing the two submenu items "Show namespace" and "Hide namespace" with one single submenu item.

In addition this submenu is only available, if the NUnit display format is checked. If another display format is checked, the submenu is not available.

If a display format is selected, the menu is not closed automatically anymore. Instead it's kept open, so that the user can select the appropriate submenu item right away.

[rowo360]

From technical point of view:
we can use the ToolStripMenuItem event Closing to suppress that the menu is automatically closed when an item is clicked. The event parameter provides some information about the closing reason to act accordingly.
This applies for the entire 'Display format' menu item now and not only the 'NUnit' menu item. But as we plan to adapt the 'Fixture list' and 'Test list' menu item too, that's also fine.
BTW: although the menu is not closed automatically, the tree view is updated to the newly selected display format right away.

[rowo360]

I decided to keep the entire logic for adding/removing the submenu in the View class. From my point of view that's a pure UI part, so involving the Presenter class for this step seems to be an overhead.