Add scroll buttons to the log window.

Not functional yet.
Added more detail for LogView arrow buttons.
Prepare for the LogView to be scrollable.
Able to actuate the buttons, nothing happens yet.
Can scroll message log now.
Added a text justification option.
Draw a label on units to indicate size.

I think we need a more general concept of a label for an entity.  The
label is drawn centered at the bottom of the entity's hex.  For units,
the label should disappear while the unit is animating.
Render text without drawing it to the screen.
Refactor UnitType into its own file.

Unit is going to become a full class instead of just a bag of bits.
Draw unit labels as separate entities.

Hide them while a unit moves.  We don't hide them during attack or
defend animations - revisit that decision later.
Slightly larger labels, clean up log wording.
New screenshot showing unit size labels.
Helper function for drawing unit size labels.
Easier way to draw numbers.