Editor Sidebar: Persist "Save Changes" panel settings

[t-hamano]

Closes #597

This PR persists the checked state of the "Save Changes" panel.

To make settings persistent, use the @wordpress/preferences package. This package is also used in Gutenberg to persist various settings, which are stored in a database. This ensures that the same settings remain the same for the same user even if the user reloads the browser or uses a different browser.

Testing Instructions

• By default, the following four items should be checked.
  • Save Fonts
  • Save Style Changes
  • Save Template Changes
  • Process Only Modified Templates
• When the "Save template Changes" is unchecked, the four checkboxes below it will be disabled as before.
• Reload your browser and confirm that the settings persist.
• Confirm that when you click "Save Changes" the theme will update according to your settings as before.

About Initializing Settings

To restart the test from the beginning, perform the following two operations.

Chrome Dev Tools > Apprication > Storage > Local storage > Right click "http://localhost:8888" > Clear

Connect to database > wp_usermeta table > Delete wp_persisted_preferences record

[pbking]

Nicely done. Works as expected. Simple change.

It's a nice cleanup for the effected view code too.

A great addition!

[bacoords]

Yes! Thanks @t-hamano (I came here to open an issue for this functionality and of course you already did it.)