Store selected kernel and toolbox in .jpblockly file

[Phoenix616]

This adjusts the format of the .jpblockly files to allow storing of additional metadata similarly to .ipynb files: It stores the selected kernel and toolbox as well as adds the ability to define a list of allowed blocks.

While improving the experience when using the Blockly editor (as one doesn't need to select the kernel and toolbox manually on each file open) this also makes it possible to distribute pre-configured files for certain types of content e.g. for exercises that should be done only with certain blocks.

Old file in the previous format are loaded and automatically stored in the new one on the next save.