[FEATURE] Add composition blend modes for layers (#5248)
Add support for loading and saving blend modes to project files
New blend modes combo box widget
Add blend modes widget to vector layer properties dialog
Add blend modes widget to raster layer properties dialog
Merge pull request #475 from nyalldawson/blend_modes

Add support for blending (composition) modes, eg Overlay, Mutiply, etc (Close #5248)