Support for class names import

[Samweli]

Fixes #823

The new Import button under the Custom Land classes group box provides an option to select a CSV file and then opens a dialog that allows assignment of parents into the new class names.

The new import button in the Land Cover tool buttons

Screenshot of a CSV file and the resulting import dialog look.

File	Dialog

[amyburness]

@Samweli Feedback from client

• Importing a LCLR example went quite smoothly.
• Import button is a little smaller than the other buttons, it would be good to make it the same size.
• LC class names are not adjusted to show full name once the dialog is expanded (see below)  

[Samweli]

• LC class names are not adjusted to show full name once the dialog is expanded (see below)

Thanks @amyburness . The reason the LC class names are not adjusted to show full name is there is a 20 character limit for class names keys, At the moment I have added logic to trim down the LC class names for those who have more than 20 characters. If this is not adequate we can discuss and handle the situation differently.