MDL-39087 Use progress_trace class to display uninstallation progress

MDL-39087 Use progress_trace class to display uninstallation progress

This is much better API than using the array passed by reference. At the
moment, it is pretty hacky as it abuses text_progress_trace to output
raw HTML echoed by uninstall_plugin() but that will be improved later
while moving the logic out of that function into the plugin_manager.