Make the ResourceId system extensible

[slodge]

Currently the resources for layouts are overriden using a LayoutId enumeration which is looked up in a hard-coded lookup map - enumeration to the string name of an Android ResourceId

This system uses context.Resources.GetIdentifier() to find the actual resource.

In the new version, we would:

• use a system of String LayoutId names
• use reflection to get the resourceId integer from the string

This would have the key advantages:

• it would allow people to register their own resourceids (this is currently "// TODO: figure out what context to use to get this right, currently doesn't inflate application resources")
• it removes the deprecated GetIdentifier mechanism

Use of this system would mean that lines like:

                    new StringElement("DialogListView Activity", (int)DroidResources.ElementLayout.dialog_labelfieldright)

would become:

                    new StringElement("DialogListView Activity", "dialog_labelfieldright")

One negative impact on this would be the use of string matching for the layoutids - I'm open to ideas on how to improve this - but the current new design is no worse than the existing one (which also uses string resource names)

[slodge]

This required huge changes in the end - changing lots of parameters from int's to string's - but it is now done - so closing :)