[processing] Small fixes

A number of fixes for small processing issues:
1. Search for description files for models and scripts also in the sub-directories of the model/script directories. This allows the users to better organise their model/script files.
2. If a algorithm provider specifies a list of valid file extensions, use it instead of the list provided by GDAL. Some providers need files output in formats not supported by GDAL when, for example, an output of one algorithm is used as an input for another in a model.
3. Add GRASS r.slope and OTB Split Image algorithms which used to be present but have disappeared. 
4. Add <html> tag when displaying R algorithm result graphs. Without it the graphs were not displaying properly. 
5. Since multi-threaded execution was removed the algorithms do not always switch to the log tab and update with the algorithm messages during execution. This fix makes sure that the log tab is at least visible so that the users know that an algorithm is being executed.
6. Update processing toolbox whenever an algorithm provider plugin is activated or deactivated from the plugin manager. Currently the update happens only when the processing toolbox is switched from advanced to simplified mode.