Brush loading + sets

[dsizzle]

• load/save a single brush from/to disk
• load/save brush set from/to disk
• default brush set, not overwritable - but allow removing the entire default brush set from the Brush Store window

currently the code saves out the brushes as a preference, and reloads on start - seems like this code can be used/refactored to implement this. For example, load all the brush sets, and then load the 'loose' brushes.