This adds three commits, making these changes:

1. Do not reset rotation on 'Center the model'. This used to also set the rotation to the 'Bottom' viewpoint, which is not always desirable. Now it only resets the scale, center and zoom to the default values.
2. Add shortcuts to Viewpoint submenu. The shortcuts are copied from PrusaSlicer, which is likely to be familiar to fstl users.
3. Add isometric view.