Fixed Theme support.

The binding `oxygen.themeManager` no longer exists because it doesn't work with lazy loading, use `Oxygen\Core\Theme\ThemeManager` instead.
Added `Oxygen\Core\Theme\ThemeServiceProvider` to `composer.json`
Reverted theme bindings
Removed `Oxygen\Core\Model` namespace.

Oxygen's `Model` namespace has been replaced with the `oxygen/data` package.
Added check for `null` theme