Refactoring. Implemented a custom Add Dialog.

The new dialog covers specific naming validations for sketch names.

The refactoring was just to have separate classes for various concerns.

Features left:
 - Open the sketch after created
 - Expand the new sketch snippet after creation.
 - Maybe: support for renames of folders and sketches so that both
items get renamed at a time. Not sure if it’s worth the effort.
Misc left:
 - Look at the TODOs
 - Move classes into their own files to defer module loading and for
cleanliness