Bug 6755 Problems with switching languages

Bug 6755 Problems with switching languages

This patch solves the situation that news is in another language than
the Koha interface AND makes that themelanguage routine is always called
the same way in order to prevent mixed display.

It fixes also a bug related to language preselection based on web
browser prefered language.

September 9: Adjusted with input of Frederic Demians.

Septembre 10: Avoid circular dependency, as pointed by Chris Cormack.
Templates related functions are moved from C4::Output to C4::Templates

Signed-off-by: Alex Arnaud <alex.arnaud@biblibre.com>
Signed-off-by: Ian Walls <ian.walls@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Chris Nighswonger <chris.nighswonger@gmail.com>