Brushes: remove selection indicator once brush settings change

[humdingerb]

You click on a stored brush, then change size/apect/etc. That's doesn't change the selected brush, but creates a new "current brush" (which could be stored as a new brush). The selection indicator of the stored brush should therefore be removed.

As collateral damage, the menu item "Delete selected brush" would then need to be disabled, too.

[dsizzle]

pulled from comments on PR #518 -

"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]

what if you change the settings and it matches an existing brush? Should that be selected? I wonder if checking all the brushes every time you change a setting could be slow, if you have a large number of brushes.

[humdingerb]

Come to think of it, there's really not much use in having a stored bush selected if you hit upon identical settings. I wonder though how best to deal with the user trying to store the brush, if there's already one with identical settings. Say, there are 12 stored brushes, the user clicks "Store brush" and expects a 13th to appear. Instead brush 7 is selected. Is that intuitive or surprising?