Add new shortcuts for block creations

[FabienRoger]

In a Jupyter notebook, adding new blocks is easy. Just press "Shift+Enter" and it adds a new block after executing the current one.
In OCB, for now, you have to right-click, new block, and link it with the previous one, which is cumbersome.

The new shortcuts which could be added:

• Shift+ Arrow to add a new block linked to the current one in the direction of the arrow and launch the execution of the current cell
• Shift+ Enter to add a new block linked to the current one to the right and launch the execution of the current cell
• Control + Enter to execute the current cell
• Something to insert a cell between two others (this is really slow with the current system)

[vanyle]

This is related to #108
The will probably be fixed in the same pull request.

[AlexandreSajus]

A button to add a linked code block under another one has been added in #266

[MathisFederico]

Alt + Shift + Down should trigger the AddBlockButton

I think we could close this issue once this is done as block insertion is already mentioned in #228 and #230

In fact, Alt + Shift + Down could also trigger insertion if there is an existing linked block once it is implemented.

[FabienRoger]

Here is the new behavior with (Alt+)Shift + Arrow: