ProjectExplorer: Forward-declare TextMark in task.h

...to cut some build dependecies.

Changes to textmark.h affected rebuilding of translation units that did
not seem related, e.g.:

 clangprojectsettings.cpp
   -> clangprojectsettings.h
     -> project.h
       -> kit.h
         -> task.h
          -> textmark.h

Change-Id: I3c1777ecf6579c68787cb4c7e86849b15979b9bf
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/projectexplorer/task.cpp

@@ -25,6 +25,7 @@
 
 #include "task.h"
 
+#include <texteditor/textmark.h>
 #include <utils/utilsicons.h>
 #include <utils/qtcassert.h>
 

src/plugins/projectexplorer/task.h

@@ -28,13 +28,16 @@
 #include "projectexplorer_export.h"
 
 #include <coreplugin/id.h>
-#include <texteditor/textmark.h>
 #include <utils/fileutils.h>
 
 #include <QIcon>
 #include <QMetaType>
 #include <QTextLayout>
 
+namespace TextEditor {
+class TextMark;
+}
+
 namespace ProjectExplorer {
 
 class TaskHub;

src/plugins/projectexplorer/taskhub.cpp

@@ -28,6 +28,7 @@
 
 #include <coreplugin/coreicons.h>
 #include <coreplugin/ioutputpane.h>
+#include <texteditor/textmark.h>
 #include <utils/qtcassert.h>
 #include <utils/theme/theme.h>
 #include <utils/utilsicons.h>