Bug 18718: Language selector in staff header menu similar to OPAC

Bug 18718: Language selector in staff header menu similar to OPAC

This patch adds a language selector menu to the top menu of staff client
similar to the OPAC.
Display of language selectors at top, bottom or both can be configured
with system preference StaffLangSelectorModer. It defaults to bottom.

To test:
- Apply patch
- Udate database
- Restart plack and memchached
- Go to staff client, verify that language selector displays at
  the bottom of the page (as before)
- Go to system preferences, verify that there is a new preference
  StaffLanguageSelectorMode (name similar to the sypref ror OPAC), and
  that it is set to 'footer'
- Change mode for top, both and footer and verify, go to staff client
  and verify for each that the language selector displays as appropriate

(Amended for comment #2  2017-06-02 mv)

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>