implement NPPM_MODELESSDIALOG as notepad->registerModelessDialog

[pryrt]

not sure whether anyone will be creating/using modeless dialogs inside this framework, but I should at least have the possibility there. Not sure how I would test it; will probably have to search github for usages of that message.

[pryrt]

PythonScript doesn't actually implement that message, and I'm not sure how I would build such a dialog in Perl (or even in PythonScript), so I'm not going to worry about implementing it for now. (Maybe something to look into for v2, when I have the plugin framework and notifications.

PS does use it at https://github.com/bruderstein/PythonScript/blob/master/PythonScript/src/PromptDialog.cpp, however.