ClangTools: Support loading exported diagnostics

Add a new toolbar button to load diagnostics exported with

   $ clang-tidy -export-fixes=/path/to/file
   $ clazy-standalone -export-fixes=/path/to/file (master version)

Change-Id: I8316fe0706a18222e68220ef4fbfdc7ae8d09804
Reviewed-by: David Schulz <david.schulz@qt.io>

tests/unit/unittest/data/clangtools/clazy.qgetenv.cpp

@@ -0,0 +1,8 @@
+// clazy-standalone -export-fixes=/tmp/clazy.qgetenv.yaml -checks=qgetenv clazy.qgetenv.cpp
+#include <QByteArray>
+#include <QtGlobal>
+
+void g()
+{
+    qgetenv("Foo").isEmpty();
+}