libanjuta: Add a priority parameter to ianjuta_editor_assist_add()

So that completions can be sorted depending on how relevent they are.
sourceview: Support the new priority parameter in ianjtua_editor_assist_add()
snippets-manager: Add provider with a low priority.
parser-cxx: Add provider with a primary priority
language-support-js: Add provider with a primary priority
language-support-python: Add provider with a primary priority.
language-support-vala: Add provider with a primary priority.
devhelp: Port to webkitgtk2

Conditionally build the plugin against the webkitgtk version that libdevhelp is
built against.
devhelp: WIP
devhelp: Work on chooser for selecting which books to autocomplete from.