Add append shim
Add overlayer shim
Make `overlayer` a property
Add gutter.getLineNumberElement shim
Use actual property for overlayer
Remove unnecessary append method.
Upgrade space-pen to make $.fn.view work with nodes appended w/o jQuery

The autosave package uses $.Event.targetView on a node that's managed
by React. This upgrade to SpacePen allows us to retrieve the containing
SpacePen view.
Add ReactEditorView::getModel
Temporary: Enable react editor for specs on this branch so we get CI feedback
Remove exceptions related to unassigned lineHeight

These exceptions break specs when simulateDomAttachment is used. They
were only present to ensure correct sequencing during development of the
react editor, so it's safe to remove them for now.
Don't assume ::component is defined
Add split methods
Add .editor class to react wrapper for package activation events
Add underlayer an overlayer classes to the correct elements
Add getFirstVisibleScreenRow and getFirstVisibleScreenRow
Add active property
Use Editor::getVisibleRowRange instead of the rendered row range

The rendered row range is larger than the visible row range to force the
DOM to break up repaints. This is a more accurate method to us.
Add ReactEditorView::getFontSize
Add ReactEditorView::setWidthInChars shim
Add the ReactEditorView::updateDisplay shim