Also introduce a indirection, modes have now types, edit and debug have
the same type, and editors do have a prefered type of modes instead of a
prefered mode.

That fixes the bug that if the prefered qml editing mode was set to
design, then in switching between qml editors would
- in Edit Mode: stay in edit mode
- in Debug Mode: switch to debug mode

* Can change files without opening an editor.
* Can open an editor without changing a file.
* Default to reindenting new files.
* Allow reading a file temporarily to access document and text.

For answers to questions about where to insert a snippet/chunk of C++ code. Ok,
currently it will only find the One And Only place to insert method
declarations in classes, and it will need some tuning.