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.
Add minor changes to name checks

Diff Tool name check minor changes - minimum number of characters is
returned to being a constant value.
Remove "close" button from Settings Form

Remove the close button from the settings form and begin changing the
behaviour of the settings and diff tool info settings forms.
Update save methods for diff tools

Updated the save method behaviour of the diff tool settings.
Fixed problem loading arguments with duplicated defaults.
Merge remote-tracking branch 'origin/development' into settings