Test suite: add generic Dogtail actions that refer to the last used application.

These are good enough (and can prevent step definition spam) when the
context is clear, e.g.:

    When I start "gnome-disks" via GNOME Activities Overview
    Then application "gnome-disks" starts
    When I click the "1.6 GB Loop Device" item
    And I click the "Menu" toggle button
    And I click the "Benchmark Disk..." menu item
    Then I see the "Benchmark" dialog
    When I click the "Start Benchmark..." button
    Then I see the "Benchmark Settings" dialog
    When I click "Start Benchmarking..." button