Keep "switch to desktop" functionnality

[ponceta]

Hi,

I'm currently testing geomapfish on ipad devices and larger mobiles screens and there is still a need to be able to redirect to the desktop version to access more functionnalities. (print, draw, share).

But it could be more discreet.

There are some/all touch pc which actually get the mobile version from GMF 1.6 as default when passing the standard url. Detection engine parameters should be improved.

"sur la 2.1 avec un notebook Win10 tactile :
-          Chrome -> version mobile + pas de lien pour retourner sur desktop
-          FF,IE et Edge -> version desktop" 

[sbrunner]

Fixed by camptocamp/ngeo#2428