Route widget update UI through Save button click event

Always attempt to re-focus last focused element
Use document.activeElement instead of manually tracking focus
Add live previews of widget changes as-you-type
Recognize builtin live-preview support for Twenty_Fourteen_Ephemera_Widget
Allow enter keypress on input to supply sanitized value even though focused
Ignore transients in an options transaction

Fix sprintf format usage
Preserve selectionStart and selectionEnd on focused input when replacing form
Add support for RSS widget and provide hooks for other widgets
Remove default classes and styles for invalid/unsanitary fields
Merge pull request #93 from x-team/live-preview-without-manual-submit

See live preview changes as-you-type, without hitting Update button