Improved text loading and saving of the NiTextBuffer.

The NiTextBuffer did not use FileType and did not correctly preserve BOM and encoding.
Force a requery after showing a modal dialog.

NiForm adds a priority command handler. To make sure that this command handler had a chance to actually validate all commands, a requery is suggested after adding this command handler.
Implemented support for delegate command mappers.

NiCommandMapper now has a Delegates collection to delegate requests to.
Minor improvements to NiTextEditor.

NiTextEditor executed ICSharpCode TextEditor commands wrong.