logic: improved include detection

* also search for include paths of include directives extracted from header search paths
* extracted logic to gather unsolved includes
* extracted some code to filetree class

bin/test/data/CxxIncludeProcessingTestSuite/test_header_search_path_detection_finds_path_included_in_future_header_search_path/a.cpp

@@ -0,0 +1 @@
+#include "a.h"

bin/test/data/CxxIncludeProcessingTestSuite/test_header_search_path_detection_finds_path_included_in_future_header_search_path/include_a/a.h

@@ -0,0 +1 @@
+#include "b.h"

bin/test/data/CxxIncludeProcessingTestSuite/test_header_search_path_detection_finds_path_included_in_header_search_path/a.cpp

@@ -0,0 +1 @@
+#include "a.h"

bin/test/data/CxxIncludeProcessingTestSuite/test_header_search_path_detection_finds_path_included_in_header_search_path/include_a/a.h

@@ -0,0 +1 @@
+#include "b.h"