Remove deprecated `g_thread_init` usage

From the docs:
  `g_thread_init` has been deprecated since version 2.32 and should not
  be used in newly-written code. This function is not longer necessary.
  The GLib threading system is automatically initialized at the start of
  your program.
Change behaviour of middle-click navigation, ignore multi-line "urls"

Rationale: The previous behaviour was extremely annoying for people (me)
using luakit on laptops with sensitive track-pads when two-finger
scrolling. Occasionally while two-finger scrolling a middle-click will
register causing people (me) to lose page position, state and form data.