Added a hook for visualization plugin to provide their demo datasets
Chart upload controller now use GET_DEMO_DATASETS hook to get the visualization demo datasets
Registered GET_DEMO_DATASETS into visualization plugin, added getDemoDataSets function
Implemented getDemoDataSets function into bar chart
Implemented getDemoDataSets function into bar chart
Removed lib/templates/demo_datasets.php - not used anymore @see GET_DEMO_DATASETS hook
Merge branch 'Plugin_DemoDatasets' into develop - GET_DEMO_DATASETS hook added & implemented