Implemented keyboard mapping options.

An options page, with support, has been added to change keyboard mappings.
Implemented priority command handlers.

It is not possible to register priority command handlers. These are called before any other command handler. The NiForm class uses this to implement modal dialogs. These modal dialogs both block all other command handling so that e.g. shortcut keys which trigger commands for other windows aren't called, and allows commands to be handled easily in dialog forms.
Made PageHost inherit from NiWindowHost`1.

PageHost inherited directly from NativeWindowHost which does not have the implementation of INiMessageFilter. PageHost now inherits from NiWindowHost`1 which fixes a number of cross-AppDomain messaging issues.
Merged NativeWindowHost into NiWindowHost.
Made ThrowOnFailure return the status of HResult.
Removed out bool parameters for INiMessageFilter and similar.