Remove any Marker usage other than for insert

[cpcallen]

The insert action should be the only way to place the marker, and when using that action the marker should be transient.

[rachel-fenichel]

#187 addresses making the marker ephemeral.

If the user has the cursor on a block and hits T to open the toolbox, I think the marker should be placed at an appropriate connection and the block selected from the toolbox should be connected if possible.

For instance, since the default for a stack block is to insert before, the marker should be placed before the block to show where the insertion will occur.

What do you think?