all: Fix namespace for attributes macros

They are only there to be used in our headers, they are not
part of our API. So prefix them with an underscore.
vteapp: Reindent

Fix vala mode indentation. And fix setup so that vala code is never
using tabs!
vteapp: Fix build with older vala

https://bugzilla.gnome.org/show_bug.cgi?id=729639