Exodus Inspector: Time slider additions

This issue was created automatically from an original Mantis Issue. Further discussion may take place here.

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1. Add labels showing ID of smallest and largest timestep, positioned at the left and right sides of the time slider.
2. Add a spin box showing the ID of the current timestep. Changing this spin box should update the current timestep in a manner consistent with the current UI mode.
3. Add a label that shows the value of the current timestep.

Check with the current version of Prism for an example of how this should work.