Make all program dropdown boxes extend to full height when clicked

Only relevant when compiled, as a manifest is required to enable this
behavior.
Dropdowns with picture box containers: fix "two-clicks-to-drop" behavior
Selection tools: lots of code clean-up

New features and code elements allow me to drop a lot of old (and now
unnecessary) selection code.  Rendering has been fully moved to GDI+, so
the old GDI fallbacks for non-antialiased selections are now gone.
Coordinate math now uses standardized PD functions.  Unneeded debug
messages removed.  Etc.