Enhance libi3 and use it in i3bar.

Abstracted draw_text and predict_text_width into libi3. Use
predict_text_width from libi3 in i3 too. This required tracking
xcb_connection in a xcb_connection_t *conn variable that libi3
expects to be available in i3bar.

src/cfgparse.y

@@ -1546,6 +1546,7 @@ font:
     TOKFONT STR
     {
         config.font = load_font($2, true);
+        set_font(&config.font);
         printf("font %s\n", $2);
         FREE(font_pattern);
         font_pattern = $2;