Add a reset view option

[matterhorn103]

Something that would be useful to have is a "Reset View" option. This should return the view to the default initial state: axes in their default alignment (+x to the right, +y upwards, +z out of the screen), the origin of the coordinate system in the centre of the view, and the zoom level at the default.

This seems to be exactly what happens automatically when switching between molecules in the Molecules pane – each is initially displayed in the default view state. If you change the view of "molecule 1" with the Manipulate Tool, then switch to another molecule and back to molecule 1, the view is (for better or worse) not the way you left it but rather completely reset. It would be good to be able to get the same result via a menu option.

This would also be necessary to implement if it was changed so that switching between molecules doesn't reset the view automatically (which would also be a good change to make, as the lack of persistent state makes comparing similar files/molecules difficult if the aspect of interest isn't easily visible in the default view).

In the View menu we already have options to "Centre" and "Align View to Axes", which are useful too in specific situations, but seem to be subtly different and don't achieve a reset, even in combination:

• Centre puts the calculated centre of the atom ensemble in the centre of the view, not the coordinates origin, and while it changes the zoom level to an appropriate one, it is not the origin that is put "in the plane" but rather the centre. It's also not clear to me how the alignment of the axes is decided, but it's not the default.

• Align View to Axes is very close to a reset, but doesn't have quite the same effect as switching to another molecule and back. The axes are reset to the default orientation, but again it seems the view is centred on the centre of the atom ensemble, not on the origin.