Fix breadcrumb rendering for all themes.

This should make the breadcrumbs bar render properly on pretty much any 3.4/3.6 theme.

-  Breadcrumbs border radius, background color, foreground (text) color, and border color are now themeable.
-  Split theme elements into two sets - fallback (for themeable elements - see src/nemo-style-fallback.css) and application for non-themeable elements (see src/nemo-style-application.css).  If I can figure out how to control cell renderers via CSS eventually I'll make the disk space indicators themeable as well.

Tested on multiple themes - adding the elements in the -fallback.css file to a user theme overrides the fallback set.