Keep only one menu item per precision, and have two separate radio
menu items for gamma/linear. Also add gamma/linear radio buttons to
the precision conversion dialog.
- change start() and set_text() to use "format" and "..." instead of
"message", allowing to format progress messages in place
- s/cancelable/cancellable/
- move "cancellable" to be the second argument of start()
For conversions that have no dither options (like RGB -> GRAY or u8 ->
u16), always preserve text editability, for conversions that have
dither options (like RGB -> INDEXED or u16 -> u8), give the user the
choice whether to enable dithering.
