Allow for global webapps

This came up at least two times recently:

it'd be cool to have 'global' webapps, that is desktop file goes to /u/s/applications and icon to /u/share/icons while all the state to the users home. This allows to distribute webapps via package management which is e.g. useful for the help pages (/cc @david.boddie)

There are several work arounds (like using a writeable dir outside /home but i think we can make this work properly. Current issue is

• epiphany expects /home/agx/.local/share/<webappdir> to exit
• it expects the desktop file in there

This can be fixed by passing the original desktop file in via ephy_web_application_setup_from_profile_directory on startup. This is only used once from main so we can pass options down easily.

This leaeves the issue that the 'delete webapp dialog' does not work for global webapps (we should detect these and then just not allow uninstalling them as is e.g. also done in firefox for extensions that come out of deb packages).