ttools: add drag zoom to other plot types

Using a modified drag gesture to zoom is now added to the remaining
plot types: sky and cube.  These are isotropic zooms, so don't confer
much advantage over the existing wheel zoom gestures, but it's worth
adding them since: (1) it's good if you don't have a mouse wheel and
(2) if people get used to doing this in other plot types it's intuitive
to have them throughout.
topcat: remove zoom in/out buttons from layer plot windows

There are better ways to zoom now.  The key bindings (removed in
an earlier commit) are also no longer required.
ttools: refactor navigation mouse listener code

Add a new class NavigationListener which translates mouse gestures into
surface re-aspecting requests.  This can now be used in both topcat
and stilts, avoiding duplicated code.