New public methods:
void clear(void);
void clear(Fl_Color val);
old protected methods made public:
void clear_screen(bool scroll_to_hist=true); // ESC [ 2 J
void clear_screen_home(bool scroll_to_hist=true); // ESC [ H ESC [ 2 J
void cursor_home(void); // ESC [ 0 H
test/terminal modified to test these, and added separate tests
for both the API and ANSI code ways to do these ops.
textcolor() needed for consistency, color() behavior documented.
Both added to the test/terminal app.
Elaborated on the special background "see through" color value + behavior,
tested with color() in test/terminal app.
