There should be an insert button on each edge

[FabienRoger]

Inserting blocks between two blocks is very cumbersome.
Desired behaviour:
A little "plus" sign in a middle of each edge that, when clicked, displays the right click menu.
When a block is selected:

• if it's a code_block, delete the edge, push the blocks below down to make place for the new block if needed, create the new block, and add edges between this block and the two blocks which were connected by an edge.
• if it's a markdown block, don't delete the edge, put push down the blocks if needed.

[MathisFederico]

Not sure edges are the right way are they are not always present where you want to insert a block.
I'm more thinking of an "Insert" option similar to spreadsheets, where you could choose to insert on the x axis, the y axis or both.

[MathisFederico]

The idea mentioned in #214 could be a potential fix also.