background layer is not selectable
disabled merge with background layer
disabled deleting backround layer
added rename layer undo
fixed delete layer undo
refactored LayerManager.qml
background color is changeable
added undo command for changing layer visible and layer lock
saved layer attributes in openRaster file
setted bg color on loading openRaster file
fixed broken eraser tool
brush and eraser actions added to edit menu
refactored clear canvas function