So that selection inside widgets isn't affected

Issue #3342

So that they don't end up overriding more specific styles.

Issue #3325

So that the contenteditable input style doesn't look silly
on desktop.

Using the CodeMirror-guttermarker and CodeMirror-guttermarker-subtle CSS
classes to give the markers their color.

Issue #2602

Closes #2415

`cm-s-ambiance` and `CodeMirror-focused` are classes to the same
element, so search for `.cm-s-ambiance .CodeMirror-focused` will fail.

Rather than adding a new rule.

Issue #1747

Added .CodeMirror to end of class names so that under CodeMirror v3, the
authors intended BG/FG is used rather than the parent styling seen

Issue #974

Fix bad rules in themes, measure width correctly.

This will make fixedGutter work properly, and make it the only model
(no more gutter that scrolls out of view).

Also reorganizes the basic css file to be easier to modify for users.

Issue #730