The state of the buttons was not changed properly

The labelfile object was not stored when saving.

The dialog switch enables adding an extension automatically.

Also pop an error message when trying to save a file with no labels.

Seems more intuitive.

The button should not track the drawing signal, it should always be
disabled in draw mode.

The highlight was not cleared properly under some conditions.

Use the same dialog for creating and editing labels.

The default cancel action when creating a shape is to undo the last
line, since this is less destructive. The user has the option of
deleting the shape after it is created anyway.

Add edit/copy/delete actions to menu, so that their shortcuts are
available as well.

Copy line/fill color when duplicating shapes.

Fix off by one error in double click event handler and refactor closing
and finalising code.

Also remove delete from the advanced mode.

The selection is now updated correctly even when moving inside the same
shape.

Also fix a bug when loading files whereby the shapes were not properly
closed.

Also remove zoom controls from toolbar and replace them with hide/show
controls.