VCS: Replace QToolButtons in VCS toolbar with QActions

Change VcsBaseEditorParameterWidget to add actions/widgets to an
existing toolbar, instead of being a widget itself.

The class is renamed in a follow-up commit.

Task-number: QTCREATORBUG-14934
Change-Id: I473a439d12a096f4cbb64f06faa0598ee72000de
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/plugins/texteditor/texteditor.h

@@ -190,6 +190,7 @@ public:
     using QPlainTextEdit::cursorRect;
     QRect cursorRect(int pos) const;
     void setCursorPosition(int pos);
+    QToolBar *toolBar();
 
     void print(QPrinter *);