Assign default tool on first tool setup

Add static method to DiffToolInfo class to determine the default tool
from a given list. Use this to determine when creating a new tool, if
there is no current default to identify the new tool as default.
Update add extension and arguments entry

Add Extension and Add Argument now, on valid entry, clears the
associated textbox entry and returns focus appropriately.
Add default arguments to diff tool

Add two essential default arguments to the diff tool class.
Also add a hard-coded list of default recognised arguments to the
diff tool info edit form.