Revert "QT5 -- QWT 6.1 canvas fixup"

This reverts commit 54234ceca460dd4772a50f3b40d0d3d556418176.
QWT 6.1 canvas() revert and LTM ToolTip fixups

Reverted the update to the QWT 6.1 code to make QwtPlot::canvas()
return a QwtPlotCanvas -- it now returns  QWidget.

This means our local copy of Qwt is the same as the published version
so we should be able to stop maintaining our own copy when Uwe pushes
the multiaxis stuff with 6.2.

Also fixed the LTM tooltip - the zoomer has been removed.
Fix xBottom lost in AllPlot

.. when you select or deselect a data series.
Data Series not Channels

.. thats a term they use in WKO+ not GC !