Fixes #1501 Add method on IActivePanel to give visualizer control of inner toolbar

[pmeijer]

See graphviz panel for example of how to add own btns and overwrite look of toolbar and buttons.

The dropdown menu now also contains maximize/exit maximize allowing the visualizer to completely disabled the potentially blocking toolbar. Simply overwrite:

MyVizPanel.prototype.getSplitPanelToolbarEl = function () {
    return null;
};