BrushStoreWindow: don't allow duplicate brush adding

[dsizzle]

also fixed selection of brush on deletion, as well as moving between brushes with left/right arrow keys.

fixes #502

[humdingerb]

Nice touch with the selecting with cursor left/right!

Some things though:

1. Deleting the last remaining brush will crash:
   ArtPaint-1424-debug-07-05-2023-06-25-29.report.txt
2. Do this:

• Store a few brushes
• (You may need to select some other brush than the first, first)
• Select the first brush
• "Delete selected brush" until none (or one, beware above crash)
  --> There's an outline left from a deleted brush-square:
  

4. "Store brush" should immediately select that just stored brush.
5. "Reset brush" should remove any selection from the stored brushes (except if you have stored a brush with the "Reset" settings, then it should be selected).

[dsizzle]

items 1-4 should be fixed.

"Reset brush" should remove any selection from the stored brushes (except if you have stored a brush with the "Reset" settings, then it should be selected).

I'd rather do this as part of #504 , not this change.

[humdingerb]

Nicely fixed!