Import ui

Adding the ability to import previously exported data + some UI for it.
Does ImportError really need it own class hierachy with parent Throwable? Perhaps have  it inherit from Throwable or is Exception (preferred) or Error more appropriate.

Similarly, the naming of UnexpectedImportDataFormat.java should follow. Throwable subclasses should indicate which subclass they belong to, e.g - UnexpectedImportDataFormatException.java