Add R_LIBS_USER option for windows

Hopefully a more proper attempt to fix http://hub.qgis.org/issues/11603 directly in processing by means of providing an option to define a path for user libraries on MS Windows. Not sure if this could be useful for other OSes too?

Author: ninsbl

python/plugins/processing/algs/r/RAlgorithmProvider.py

@@ -61,6 +61,9 @@ def initializeSettings(self):
             ProcessingConfig.addSetting(Setting(
                 self.getDescription(),
                 RUtils.R_FOLDER, self.tr('R folder'), RUtils.RFolder()))
+            ProcessingConfig.addSetting(Setting(
+                self.getDescription(),
+                RUtils.R_LIBS_USER, self.tr('R user library folder'), RUtils.RLibs()))
             ProcessingConfig.addSetting(Setting(
                 self.getDescription(),
                 RUtils.R_USE64, self.tr('Use 64 bit version'), False))
@@ -70,6 +73,7 @@ def unload(self):
         ProcessingConfig.removeSetting(RUtils.RSCRIPTS_FOLDER)
         if isWindows():
             ProcessingConfig.removeSetting(RUtils.R_FOLDER)
+            ProcessingConfig.removeSetting(RUtils.R_LIBS_USER)
             ProcessingConfig.removeSetting(RUtils.R_USE64)
 
     def getIcon(self):