Allows convenient showing of panels (DOM nodes) above or
below an editor, automatically shrinking the editor to make
space.

Closes #2598

Bumping them for every release just adds too much git
noise and history blowup.

Issue #2439

Most modern browsers now keep displaying the placeholder even if the
user has focus on the input and only hide it when the input has content.

This change is to make this addon consistent with this, so users won't
tell the difference with native inputs' placeholder behaviour.

Issue #1761

Issue #1761

Rather than polluting the top-level of the editor object