Remove conditional compilation for cairo/pangocairo (#2480)

We strive to avoid conditional compilation in i3 as much as possible.
cairo and pangocairo have been around long enough in the versions that
we need that it’s time to unconditionally depend on them.

Also update DEPENDS with the last-known-good-versions while at it.

libi3/string.c

@@ -13,9 +13,7 @@
 #include <stdlib.h>
 #include <string.h>
 
-#if PANGO_SUPPORT
 #include <glib.h>
-#endif
 
 #include "libi3.h"
 
@@ -192,15 +190,11 @@ void i3string_set_markup(i3String *str, bool pango_markup) {
  * Escape pango markup characters in the given string.
  */
 i3String *i3string_escape_markup(i3String *str) {
-#if PANGO_SUPPORT
     const char *text = i3string_as_utf8(str);
     char *escaped = g_markup_escape_text(text, -1);
     i3String *result = i3string_from_utf8(escaped);
     free(escaped);
     return result;
-#else
-    return str;
-#endif
 }
 
 /*