adding Display and RegularExpression atts

This means we can do regex validation on each property through the
RegularExpressionAttribute instead of relying on the object-level
Validate method to perform it.  This helps set us up for better in-field
validation for individual properties.
adding icon to application
adding splash screen image
adding tooltips to fields (close #10)

There are static dictionaries, populated by the sprite sheet VM class
constructor, which use reflection to iterate over all public properties
on the sprite sheet/sheet types and add entries based on the Description
of the DisplayAttribute applied to those properties.

These dictionaries are exposed as ReadOnly properties and the ToolTip
attribute of each relevant text field is bound to the appropriate
dictionary entry.