Rename pane-clear to Draw:clear

Rename pane-clear to Draw:clear

This is part of making all commands for changing the display start with
Draw:.

- pane-clear is now Draw:clear
- str2 is ignored, str1 has the attributes
- render-present expected "color:" to be followed by a colour, not
  "bg:colour".  This requires the test to be refreshed.
- add some documentation to "DOC/Calls".

Signed-off-by: NeilBrown <neil@brown.name>