Fix #113

This updates the GB hash to use the isSelected prop as an indication for the PlainText-backed blocks (currently the More and Code blocks) to gain focus when such a prop is true.

Related Gutenberg PR: WordPress/gutenberg#11989

Before, the focus would not be gotten when a new "PlainText"-backed block was inserted:

Now, the cursor appears blinking after insertion:

To test:

1. make sure to select a block (tap on a block and see the up/down arrows appear, etc)
2. now tap on the + inserter icon
3. choose one of Code or More blocks (the only ones that currently are backed by PlainText implementation)
4. observe the new block is inserted, the block is shown as selected (it has the up/down arrows and such) and the cursor starts blinking there (that is, it got the focus).