New dependency limitations

Hi,

Since elementary OS is the only major Linux OS shipping Epiphany by 
default, I think we should start catering a bit more to them. Of course 
Epiphany will still be the GNOME web browser first and foremost, but we 
can do some things to make life easier for the elementary developers:

 * I've offered to accept elementary's downstream patches upstream, 
provided they add suitable #if guards or GSettings toggles so that the 
upstream user experience is not impacted.

 * I've offered to ensure Epiphany does not depend on software newer 
than is available in next year's forthcoming Ubuntu 18.04 release. An 
exception is WebKit; we'll of course continue to unconditionally 
require new stable WebKit versions in stable releases of Epiphany 
whenever we need them. This will allow elementary to ship new versions 
of Epiphany every six months. Currently, they are shipping Epiphany 
3.18, and they're happy with it, but I think they would be happier if 
they were shipping the latest version, which they should be able to do 
beginning with Epiphany 3.28. We will just have to add #ifs when we 
need to use stuff that's not available in the Ubuntu LTS that the 
latest version of elementary is based on.

I think these changes will have only a minimal impact on our 
development.

Michael