Add new disclosure button images.

To be displayed beside folders on hover.
Add folder loading frame images.

Used by the icon_folder_loading class.
Fix file icon hover tint.
Make folder icons and disclosure buttons unique.

This will probably look worse in older builds of ST3. Be warned.
Update fold buttons to use disclosure button graphics.
Set dark_content to false for Higuchi.

This is not the case for the actual story.
Fix row padding in themes.

Made the right margin of the sidebar too small.
Add tab control buttons.
Change tab buttons to use disclosure buttons.

This also adjusts the highlight color for Kano because it wasn't
noticeable enough when it needed to be.
Add messages.json and notes for 1.4.0