Replaced "Replot" button with a toolbar action

This makes the button smaller, preventing some size issues with long
z-axis names. Also added tooltips for the whole toolbar.
Improved naming of imported data

Will now attempt to name the root after the imported file name, or, in
the case of multiple files, a common prefix or the directory name. It
uses the same algorithm as for opening NeXus files.
Added ability to handle reverse rotations

Can now define a starting rotation angle (omega), step size, and
whether it's increasing or decreasing.