Moved hard-coded text to language file (backend)

Hard-coded text from the backend (user- and group management) moved to
the language file (English and Dutch).
Updated Backend + System language support

Moved hard-coded text in "backend" and "system" to the language files
(English and Dutch). Affected pages: administrator management,
administrator group management, system updates, system settings, email
settings. Also made some strings more generic for general use (like
"Saving...." instead of "Saving administrator account...")
Updated CMS language support

Moved hard-coded text in "cms" to the language files (English and
Dutch). And some minor changes.
Merge pull request #111 from erJasp/master

Moved hard-coded text to language file (backend)