First work on layout, using more sass variables and less styling via js.

See commit comments. This is just some small first changes to get into how the project works.
General philosophy:
- The composition component contains the video canvas and can change when ui-panels are collapsed.
- Composition uses DOM elements from index.html as its 'THIS.EL', so it's clear how things are nested. We can start using templates later.
- Started to convert some values in scss to variables, to make it easy to modify dependent arrangement of elements on screen.
- Some styling that conflicted with scss had been removed from javascript.
- Nothing final yet, but it would be good to merge before we get too many things to review.