Add "test canvas" to Brush Editor

[Gazook89]

It might be helpful to have a small canvas area in the Brush Editor that allowed a user to paint a stroke on it with the current brush settings.  Each stroke would be erased on any subsequent stroke, and they would not be saved if the brush editor window is closed.  This could be in addition to the brush preview that already exists at the top of the editor, or it could grouped with it (in perhaps a 'tab' UI component), or the current preview could be the "initial" stroke/live preview until you click on it, at which point it is a test canvas-- then, making any changes to the sliders or other options would reset it to the preview, ready for the next test stroke.

For me, this would save me from filling a layer up on my canvas with test strokes before deleting the layer each time it fills up. I could work on brush editing in a single screen, rather than in the editor dialog and on the main canvas.