VCS: fix VCS pane open

Version Control output pane creates only when project with VCS support
opens. So, registration in outputpanemanager.cpp::g_outputPanes makes
with some delay and after OutputPaneManager::init() call. As a result,
g_outputPanes size and internal m_buttons size is differ and impossible
to enable VCS output pane in GUI.

Task-number: QTCREATORBUG-19971
Change-Id: Ifb8a759fdbf39484c8648dd8ecc0b3f573cf45eb
Reviewed-by: hjk <hjk@qt.io>

src/plugins/vcsbase/vcsplugin.cpp

@@ -130,6 +130,9 @@ bool VcsPlugin::initialize(const QStringList &arguments, QString *errorMessage)
             return QString();
         });
 
+    // Just touch VCS Output Pane before initialization
+    VcsOutputWindow::instance();
+
     return true;
 }