Added:
- AMExportController::scanFromUrl() ~ Function which takes a scan URL
and returns the scan (or 0 if no scan can be loaded from that URL)

Altered:
- AMExportController::AMExportController() ~ Now both constructors
determine the default export option by checking the default for the
first scan. (Assumes that all the scans are the same as the first one)
- AMExportWizardOptionPage::populateOptionSelector() ~ Now selects the
option for the combo box depending on the export controller's current
default option.