doc: fix docstrings in urwid.Widget
doc: fix docstrings in urwid.Widget

..which resulted in a few sphinx warnings.
this also adds some TODOs

hkoof/urwid,inducer/urwid,harlowja/urwid,urwid/urwid,urwid/urwid,zyga/urwid,rndusr/urwid,foreni-packages/urwid,hkoof/urwid,bk2204/urwid,foreni-packages/urwid,rndusr/urwid,inducer/urwid,drestebon/urwid,harlowja/urwid,tonycpsu/urwid,wardi/urwid,harlowja/urwid,douglas-larocca/urwid,bk2204/urwid,drestebon/urwid,hkoof/urwid,westurner/urwid,mountainstorm/urwid,inducer/urwid,douglas-larocca/urwid,mountainstorm/urwid,wardi/urwid,urwid/urwid,ivanov/urwid,tonycpsu/urwid,rndusr/urwid,tonycpsu/urwid,mountainstorm/urwid,Julian/urwid,drestebon/urwid,westurner/urwid,bk2204/urwid,foreni-packages/urwid,ivanov/urwid,westurner/urwid,zyga/urwid,ivanov/urwid,douglas-larocca/urwid,Julian/urwid,wardi/urwid,zyga/urwid,Julian/urwid