ttools: add graphics for old-style 'frame' zoom to plot2 navigators

The frame drag zoom that was the main way to zoom in the classic plot
windows has its enthusiasts.  This commit adds the rubber-band
graphics for implementing this to the plot2 windows, using drag with
mouse button 2 (or shift).  The actual associated zoom actions are not
yet present.
ttools: refactor mouse event decoding logic

Move the logic that determines mouse position and button index from
individual Navigator implementations to NavigationListener.
Makes it easier to ensure that things are done consistently.
ttools: add old-style frame zoom functionality to plot2 windows

The PtPlot-style zoom GUI where you drag out a band and on drag
termination the plot is re-drawn to cover the given region is
now added, it uses the middle mouse button (or shift) drag in
plane, time and sky plots.