So that selection inside widgets isn't affected

Issue #3342

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

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

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

It's tiny anyway, no need to force people to include yet another file.

Signed-off-by: Jan Keromnes <janx@linux.com>

This normalizes the classes used by most of the modes
(reStructuredText and diff didn't fit in), and makes them all
reference a central style sheet (under theme/). It also adds a theme
option that can be set to change the theme, and defines a few
different themes (some based on old mode colors).