Issue #400: Toggle on-canvas text editor visibility

This patch attempts to resolve the main part of issue 400.

Users working on small images or text near an important detail often wish they could hide the on-canvas text editor (temporarily or permanently). @schumaml suggested in this comment that a toggle could be added to the text options toolbox to let the user control the visibility of on-canvas text editor. An example of the toggle in action is shown here:

GIMP_-_Hide_On-Canvas_Text_Editor

The default setting is "on" for discoverability and to prevent issues with user's current workflows.