widgets/knob: different deltas depending on modkeys
first crack as a text input box
replace inherited typecasts with RTB_AS_TYPE macro
implement rtb_text_input_set/get_text
implement RTB_OBJ_FLAG_EVENT_SNOOP

also, make examples/test a little more interesting.
c99 fix for clang (U"<string>" -> L"<string>")
implement RTB_UPCAST and RTB_DOWNCAST
use RTB_OBJECT_AS in SELF_FROM macros