[processing] Search for description files also in the sub-directories of the script/model directory
[processing] Allow algorithm providers to set the allowed output raster extensions if needed
[processing] Add some missing algorithms
[processing] Fix problem with displaying R graphs
[processing] Make sure the log tab is visible before executing an algorithm
[processing] Update Processing toolbox when a provider is added or removed
Merge pull request #1389 from radosuav/small_processing_fixes

[processing] Small fixes