topcat: add framework for explicit plot geometry setting

Add hooks to allow a PlotPosition object to be supplied from the GUI.
This can provide explicit settings for the size in pixels of the
generated plot graphic, as well as the borders between the exterior
of the graphic and the plot data region.  Any or all of these items
may be omitted, in which case defaults are calculated and used.

At this commit, no corresponding GUI has been implemented.
topcat: add GUI for explicit plot geometry setting

Each plot2 window Axes control now has a new Size tab to allow the
user to explicitly set some or all of plot image dimensions and
insets in pixels.
topcat: document GUI for explicit plot geometry setting

Add sections to SUN/253 about new Size panels in axis controllers.