Add more placeholders

To add more flexibility in grid manipulation by the user, each "module"
cell is wrapped in a "grid" cell with placeholders, allowing to easily
have, for example, a row with one part with a module taking the full
height, and another part with wo modules sharing the full height
Correct handling of some jsx files
Add plugins to jsdoc

- summarize, that will extract a summary from the description
- markdown, to ease render complicated descriptions, returns...
Render the app in a div, not in body
First real grid rendering