Add option to restore last session at startup

* Add an option to the session manager to restore the last
   session on startup of creator.
 * Align close button on session manager dialog with the other
   buttons.
 * Clean up handling of session restoration in the ProjectExplorer:
   Move all the relevant code into determineSessionToRestoreAtStartup,
   since most was there already.

Task-number: QTCREATORBUG-2324

src/plugins/projectexplorer/sessiondialog.cpp

@@ -149,6 +149,17 @@ SessionDialog::SessionDialog(SessionManager *sessionManager)
     markItems();
 }
 
+void SessionDialog::setAutoLoadSession(bool check)
+{
+    m_ui.autoLoadCheckBox->setChecked(check ? Qt::Checked : Qt::Unchecked);
+}
+
+bool SessionDialog::autoLoadSession() const
+{
+    return m_ui.autoLoadCheckBox->checkState() == Qt::Checked;
+}
+
+
 void SessionDialog::addItems(bool setDefaultSession)
 {
     QStringList sessions = m_sessionManager->sessions();