Update gimp ports to use webkit2-gtk

[jeremyhu (Jeremy Huddleston Sequoia)]

Please update gimp2, gimp2-devel, and gimp3-devel to use webkit2-gtk rather than webkit-gtk.

The older version of webkit-gtk hasn't been supported upstream for quite some time and is full of vulnerabilities.

See #52398

[dbevans (David B. Evans)]

gimp2 and gimp2-devel are based on gtk2 and could not be ported to webkit2-gtk without porting to gtk3 first. There is an upstream git branch that is working on the gtk3 update targeting a gimp 3.0 release some time in the future. gimp3-devel is a port of this branch which currently uses webkit-gtk3. But looks like gtk3 support is a ways off yet.

The good news is that webkit-gtk is only used to implement a help browser which can be disabled using --without-webkit. The help browser is conditionally built based on the variant +help_browser. If webkit-gtk is removed, the easiest solution is to just remove this variant from the ports. Without the help browser enabled, the gimp ports use an external web browser to display online html docs so nothing lost really.

[dbevans (David B. Evans)]

Changes committed in r153690 that remove the +help_browser variant and its corresponding dependency on webkit-gtk, webkit-gtk3. Default preferences are updated to access GIMP's online help docs using an external web-browser such as webkit2-gtk based epiphany for which a runtime dependency is added.