Don't break activity startup if .info have 'class' instead of 'exec'
Don't break activity startup if .info have 'class' instead of 'exec'

When we ported sugar to Gtk3 we removed the support of the fields
exec and service_name due they were deprecated for a long time [1].
But this is still a problem in deployments, and we already added
compatibility for the service_name field.
Here [3] there is a long thred were we discussed this issue.

[1] ae85dd62d42a7d5bea76aec3b941ce35ccc61bf4
[2] 32f97b48c31f60f306854753a3df734f63b798fc
[3] http://lists.sugarlabs.org/archive/sugar-devel/2013-October/045404.html

quozl/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,Daksh/sugar-toolkit-gtk3,Daksh/sugar-toolkit-gtk3,Daksh/sugar-toolkit-gtk3,godiard/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,godiard/sugar-toolkit-gtk3,tchx84/debian-pkg-sugar-toolkit-gtk3,tchx84/debian-pkg-sugar-toolkit-gtk3,tchx84/debian-pkg-sugar-toolkit-gtk3,puneetgkaur/sugar-toolkit-gtk3,puneetgkaur/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,godiard/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3,puneetgkaur/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3