Refactor dropTA to use panel functionality

[erichung0404]

Since S4 method and chartSeries.chob are deprecated, dropTA, listTA and get.chob functions are refactored to use panel functionality of xts::plot.xts and coordinate with the refactored chartSeries and add* functions.

The way TAs are removed is based on the frame they are added to. If an indicator is drawn on the price series, frame=2 or frame=-2, for example, its action will be set to NULL; otherwise plot_object$remove_frame() is called to remove the new frame created by the indicator and reset asp.