implemented cancel feedback in project saving

The cancel button now sends a signal to the saving thread to stop.

app/project/projectsavemanager.cpp

@@ -1,7 +1,8 @@
 #include "projectsavemanager.h"
 
 ProjectSaveManager::ProjectSaveManager(Project *project) :
-  project_(project)
+  project_(project),
+  cancelled_(false)
 {
 
 }
@@ -11,6 +12,10 @@ void ProjectSaveManager::Start()
   int prog = 0;
 
   do {
+    if (cancelled_) {
+      break;
+    }
+
     prog += 10;
 
     emit ProgressChanged(prog);
@@ -20,3 +25,8 @@ void ProjectSaveManager::Start()
 
   emit Finished();
 }
+
+void ProjectSaveManager::Cancel()
+{
+  cancelled_ = true;
+}