This standardizes the header inclusion guards to #pragma once, which
shouldn't be a problem as we already were using it in some files.

This makes gtk-doc ignore the macro which avoids these warnings (and
dead index entries):

    Building documentation for libhandy
    html/api-index-full.html:265: warning: no link for: "HANDY-INSIDE:CAPS" -> (HANDY_INSIDE).
    html/api-index-full.html:265: warning: no link for: "libhandy-handy" -> (handy).

The widget prints a number of arrows one by one to indicate a sliding
direction. Number of arrows and animation duration are configurable.