cosmetic chages in QGISAlgorithm provider, also remove unused icons and
add new ones to resources
review and cleanup Convex Hull tool, also move some common routines to
utils module and update other algs accordingly
review Clip tool
review Difference tool
housekeeping: remove obsolete code, completely switch to new vector API
[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)