screen: Add helpers to easy use color by name

Added macros which allow easy specify color pair like:

    INIT_COLOR(YELLOW, BLACK);
    COLOR(YELLOW, BLACK);
    COLOR_ON(YELLOW, BLACK);

by calculating pair id via generic formula. Added shorter
color names via new enum.

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

screen.h

@@ -3,6 +3,23 @@
 
 #include <curses.h>
 
+enum colors {
+	BLACK   = COLOR_BLACK,
+	RED     = COLOR_RED,
+	GREEN   = COLOR_GREEN,
+	YELLOW  = COLOR_YELLOW,
+	BLUE    = COLOR_BLUE,
+	MAGENTA = COLOR_MAGENTA,
+	CYAN    = COLOR_CYAN,
+	WHITE   = COLOR_WHITE,
+};
+
+#define COLOR_MASK(fg, bg) ((fg) + (bg) * (COLOR_WHITE + 1))
+#define COLOR(fg, bg) COLOR_PAIR(COLOR_MASK((fg), (bg)))
+#define INIT_COLOR(fg, bg) init_pair(COLOR_MASK((fg), (bg)), (fg), (bg))
+#define COLOR_ON(fg, bg) attron(COLOR(fg, bg))
+#define COLOR_OFF(fg, bg) attroff(COLOR(fg, bg))
+
 extern WINDOW *screen_init(bool israw);
 extern void screen_end(void);