Allow for deselection of HighlightedValue. Fixes in BaseApp.

[slootsjj]

This change includes the following:

For ListSelectorTwoPane and ListSelector:

• Allows for deselection of HighlightedValue in ListSelectorTwoPane which was previously not possible due to a change because of issue #57. Please test if the current change does not re-introduce issue #57.

For BaseApp:

• Fixes issue #68
• Uses arguments block for input parsing
• Uses isprop(obj.Figure, "Position") instead of isvalid(obj.Figure). If a user deletes the figure instead of the app, the delete method of the app will still be triggered. Although not yet fully deleted (property values are still available), the app and figure will not be valid at this point.