StatusView: don't notify about brush preview until after brushes are loaded

[dsizzle]

This is a follow-on to #647 - prevents StatusView from drawing the brush preview until after they are loaded.

[dsizzle]

@humdingerb did you ever test this one?

[humdingerb]

I think I did, but should say so directly instead of just rambleabout speedometer resetting hands at ignition...

I now did once more and didnotice something:
The Brushes window appears to always have the last stored brush selected on launch. The status bar preview of the canvas window, however, always shows the 5th stored brush (for testing purposes I currently have 13 stored brushes). One time it also was the 4th brush (maybe something was slowing things down and ArtPaint didn't get to brush nr.5?). And once I got a crash:
ArtPaint-3834-debug-15-11-2023-12-28-45.report.txt

Even when creating an empty canvas, the brush shows stored brush nr.5. When drawing into the canvas, the actual brush isn't nr.5, but the one selected in the Brushes window.
Only when loading an image, the canvas brush preview jumps to the last brush, which has been selected in the Brushes window all this time.

[dsizzle]

That's weird. Mine always shows the last brush in the Status view. I had 15 brushes, and kicked it up to 18, and then 19, and still no issues. Maybe I'll try doubling the number or something.

[humdingerb]

With a 2.3GHz i7 which is in its 9th year of service, it's no speed demon, but still plenty fast. Funnily enough, if I disable 7 of the 8 cores with ProcessController and start something CPU intensive like GLTeapot without FPS limit and a few pots spinning, ArtPaint gets further in the list of stored brushes.