Make all required includes explicit

Information as seen by IWYU 0.12 + clang 9 on Linux

ScreenManager.h

@@ -7,12 +7,15 @@ Released under the GNU GPLv2, see the COPYING file
 in the source distribution for its full text.
 */
 
+#include <stdbool.h>
+
 #include "Action.h"
 #include "Header.h"
 #include "Panel.h"
 #include "Settings.h"
 #include "Vector.h"
 
+
 typedef enum Orientation_ {
    VERTICAL,
    HORIZONTAL