MSA color scheme removing can't be cancelled

1. Open "data/samples/CLUSTALW/COI.aln".
2. Ensure that there is a single menu item (Create new color scheme) in the {Colors -> Custom schemes}

   submenu of the context menu. Click it.

3. Create a new color scheme, accept the preferences dialog.
4. Ensure that the new scheme is added to the context menu. Call the preferences dialog again.
5. Remove the custom scheme and cancel the preferences dialog.
   Expected state: the scheme is presented in the context menu, it is shown in the preferences dialog.
   Current state: the scheme is presented in the context menu, but it is not shown in the preferences dialog.

Any actions in the preferences dialog should be cancelable.

The combobox on the "Highlighting" options panel widget also should show the adequate state.