fix(developer): TframeTextEditor.SetText was not synchronous

Fixes #5095.

If the text editor was loaded, `SetText` would not be synchronous
because it executed some Javascript to make the text change, so setting
the text and immediately reading it again would give the old text. This
is what the On Screen Keyboard editor was doing (it would synchronize
the visual editor and text editor at save). This fix updates `SetText`
to update the backing store as well as the front end editor.