vcsbase: add "exec command" mechanism to EditorParameterWidget

Based on BaseGitArgumentsWidget, it provides an "arguments changed"
handler that calls executeCommand() by default.
This handler can also further load/save settings

Change-Id: I7182ddfa6323205f2ee57a7ce67122be5b89102e
Merge-request: 331
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>
Reviewed-on: http://codereview.qt.nokia.com/253
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>

src/plugins/vcsbase/vcsbaseeditorparameterwidget.h

@@ -66,6 +66,10 @@ public:
     static QString msgIgnoreBlankLinesLabel();
     static QString msgIgnoreBlankLinesToolTip();
 
+public slots:
+    virtual void executeCommand();
+    virtual void handleArgumentsChanged();
+
 signals:
     // Trigger a re-run to show changed output according to new argument list.
     void argumentsChanged();