libi3/font: Set DPI for the pango context

The pango font specification accepts a font size in points, but pango
defaults to a DPI of 96. Create a default PangoContext (which
internally creates a default PangoCairoFontMap as usual) and set the
DPI to the value of the root Screen manually.

Fixes #1115
refactor previous commit’s new code into a function, add log message
Command 'move <direction>' moves across outputs

When 'move <direction>' is issued in the context of a container that
borders a workspace, and there is no suitable place within this
workspace for which this container can move, move the container to the
closest output in this direction instead.
t/516-move: use window id, fix typo