ClangTools: Add "go to project settings" toolbar button

With the introduction of the "Analyze Current File" action the widget to
configure the diagnostic config was moved to the project panel (Project
mode > Project Settings > Clang Tools).

As not too many users are aware of the project settings there and
navigating there involves more user interation now (mode switch, looking
for the "Clang Tools" item), add a toolbar button as a shortcut.

Change-Id: I4c864045ef41ff501d925d3175ce604def213f29
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/clangtools/clangtoolsconstants.h

@@ -28,6 +28,8 @@
 namespace ClangTools {
 namespace Constants {
 
+const char PROJECT_PANEL_ID[] = "ClangTools";
+
 const char RUN_ON_PROJECT[] = "ClangTools.RunOnProject";
 const char RUN_ON_CURRENT_FILE[] = "ClangTools.RunOnCurrentFile";