Added Widget.widgetAt() method.

This returns the widget at a specified screen position, optionally traversing up to an ancestor with a specified type.
Fixed TabbedContainer to hide corner widget when tabs are hidden.

The current approach might not interact well with other code which wishes to manage the visibility of the corner widget - for instance it might already be hidden when the tabs are hidden, and then get shown in an unwanted manner when the tabs are shown. The solution would be to stash the visibility when changing it due to tab visibility, but I'd prefer to wait until we have a use case before figuring out the details of that.
Added ability to hide tabs in layouts.

This is achieved via an item on the layout menu, and a Ctrl+T shortcut, which is necessary for when tabs have been hidden and the menu is no longer accessible. This will be of use for building a better standard layout including a Timeline.
Merge pull request #489 from johnhaddon/tabVisibility

Ability to hide tabs in layouts