Design mode/Qt Designer: clean-up Part II: Fix undo.

Re-introduce FormEditorFile that delegates dirty handling to the form
window. Change DesignerXmlEditable to be an IEditor that embeds
TextEditable so that the PlainTextEditor can work with it, but delegates
relevant functionality to FormEditorFile.
Centralize all form window creation code that was scattered around
in FormEditorW::createEditor() and have that return a struct Editor
data, which is passed to FormEditorStack.
Update the text editor only on open/createNew/switch away from design
mode.