qbs build: Improve some base items

Clean-up, simplification and modernization.
Also fix the bugs uncovered by this.

Change-Id: Ia3a77557f10faf9a901540a55fd6c07a2e986bf5
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/coreplugin/actionmanager/actioncontainer.cpp

@@ -4,8 +4,8 @@
 #include "actioncontainer_p.h"
 #include "actionmanager.h"
 
-#include <coreplugin/coreconstants.h>
-#include <coreplugin/icontext.h>
+#include "../coreconstants.h"
+#include "../icontext.h"
 
 #include <utils/qtcassert.h>