Cursor should always focus to left of placeholder

[madeleineostoja]

Right now the placeholder is treated as a single node, which the placeholder can be placed to the right of. This means that if you tap the editor, or delete the content in the editor, the cursor often ends up to the right of the placeholder, which looks all kinds of broken.

A few hacky patches were done in #16, but that's certainly not a long term fix. One use case (deleting content) isn't resolved at all, and others have FOUC flashes as the cursor is shoved back to the left.

This seems to be a regression from when ProseMirror was bundled with simpla-text, so worth exploring what's changed.

[bedeoverend]

So just been checking out the old simpla-text and looks like it still had most of the same problems, except that when removing all content it did reset the cursor to the left - though was still a FOUC.