ttools: navigation gestures now yield NavAction not aspect objects

This change adds a hook which allows plot navigation gestures to return
a GUI decoration with a visual indication of the navigation action
that is in progress as well as the instruction to change the surface
view.  This may make it more obvious from the GUI how to drive the
navigation.  At present, only the hooks are present, no actual GUI
decorations are implemented.
ttools: navigation decorations are now consumed as well as produced

The current Navigator classes still do not produce any decoration
icons associated with navigation actions, but if they did they would
be displayed in stilts and topcat plots.
ttools: implement navigation visual feedback for 2d plot types

Plane, time and sky plots now have working visual feedback decorations
for navigation actions.  Cube not yet done.
topcat: add an option to toggle navigation visual feedback on/off
ttools: add visual feedback for 3d navigation
topcat: deprecate old-style plot windows in the GUI

The main plotting controls visible in the toolbar are now the new-style
(plot2) windows.  The old ones are relegated to the Plot menu.
Plot window names and control actions have similarly been adjusted.

Placeholder entries have been introduced in SUN/253 for the two
new(ish) plot2 windows, histogram and time.
However these currently lack content.
topcat: improve plot progress toggle icon
topcat: added a little plus sign to control action icons

Plot2 actions which add items to a stack of controls now have a little
plus sign added to their action icons.  Also reword the names and
descriptions slightly.  This (hopefully) makes it clearer what those
actions are there for.
topcat: rename File menu to Window menu in AuxWindow

The File menu had nothing to do with files except in the Control Window,
so in all other windows it is now renamed as Window, which is much
more appropriate.  Updating all the screenshots was a bit boring.
topcat: move plot2 stack control toolbar buttons

The buttons for adding and removing controls to the control stack in
plot2 windows have been moved out of the main toolbar and into a
new sub-toolbar just above the control stack.  This is a more logical
and intuitive place to find them, especially when the control panel
is floated away from the plot display itself.  Secondary considerations:
it uses up more vertical pixels, which are at a premium especially
on laptop screens (bad), but it also gives more room for expansion
for more different layer types, which hopefully will be added in the
future (good).
topcat: set plot progress logging true by default