New Symbol Definition validation rules

[jng]

Add the following rules to check when validating Symbol Definitions:

• Check for non-existent symbol parameters
• Check that at least one geometry usage context has been specified.
• If using resource data for an image graphic, verify the resource data specified checks out
• If using inline PNG base64 content for an image graphic, verify the content checks out as an image
• If using a composite rule in a layer definition, verify that symbol references point to valid symbol definitions
• If using a composite rule in a layer definition, verify that the overrides specified are for symbol parameters that actually exist
• If using a composite rule in a layer definition, warn if the symbol definition has usage contexts that are irrelevant against the layer's feature source (eg. Line Usage context specified in a point-only feature source)

[jng]

Fixed r6394