provide a route to get the default document targets

Change-Id: Ia163d2a69b33c18e18f81f0f82855d1caa4c2c1a
tweak conversion a bit

a) rework to remove the need for a ::Resize impl
b) move the explicit pixel size into relative APP_FONT size
c) lock down the target combobox size so it doesn't jump around

Change-Id: Iad995453c178234d1ad0f794b5193920dcd275fe
drop unused helpids

Change-Id: I385fcf216645a79d5c6cdadd31a37be20938371a