Add slider for adjusting line width
Support line width setting in line brush
Add color scale slider
Streamlined geter and setter in ImpressionistUI
Support color scaling in brush
Increase the maximum color scaling factor to 500%
Refactor Brush* methods for future extensibility
Add StateManager to handles undo/redo
Initialize StateManager
Save painting before drawing
Add Undo button in menu bar
Merge branch 'master' of https://github.com/nkming2/Impressionist.git