theme: restructure the SASS and land design fixes

This merge request is two-fold:

Refactor the SASS

1. Organization and cleanup

• rewriting the SASS to be modular–makes editing and finding style definitions for specific parts of the shell easier
  • widgets each get their own scss file in widgets which are imported via _widgets.css
  • common styles and global values are put in _common
• make use of more SASS variables so there's fewer values to track; new variables for:
  • border radii
  • padding, spacing and margins
  • uncommon colors
• nest classes as much as possible as to mimic it's UI structure
• be consistent with our use of functions
• be consistent with units

2. Support the light variant

• where needed, insert all the needed $variant == 'light' styles

3. Iron out some minor visual bugs

• popovers > menus: menuitems padding and borders
• sliders: fixes issues with the borders and legibility of the handle
• keyboard: use buttons draw function for the keys
• keyboard: fix the icon assets canvases and minify
• appgrid: use button drawing for the frequent|all
• overview: use osd styling for the dash and workspace switcher
• search entry: add missing hover styles
• telegraph the media controls better with hover styles

Land some pure-CSS Design Updates

Calendar/Notifications

• consider notifications "buttons" and use that drawing function
• update use of typography
• use the button drawing for the calendar sections
• use larger icons in the notifications

Search Results

• draw backgrounds for the search sections to improve legibility of search results, fixes #288 (closed)