move themes code into OpenProject's library
fix a wrong css class regarding themes
refactor themes backend
* extract theme finding code from the Theme class into a ThemeFinder module
* omit a trailing 'Theme' in the class name for default identifier and name attributes of themes
* remove the default? method
* do not rely on AS::DescendantsTracker anymore, although it was nice
let's call the default theme 'OpenProject'. alternatively one could have changed the class DefaultTheme to OpenProjectTheme and let that be the default theme.
Remove (hopefully) superfluous TODOs

[ci skip]
Merge pull request #230 from opf/feature/rails3-themes-refactoring

Refactoring of Themes Backend