Add libi3/load_font, use it everywhere

…except for i3bar, which needs slightly more information about the font

i3-config-wizard/xcb.h

@@ -11,6 +11,5 @@
 extern unsigned int xcb_numlock_mask;
 
 xcb_window_t open_input_window(xcb_connection_t *conn, uint32_t width, uint32_t height);
-int get_font_id(xcb_connection_t *conn, char *pattern, int *font_height);
 
 #endif