Convert loadFileWaitTillLoaded to a promise

In 2.11.3.3 we occasionally get deadlocks in tests due to off-awt thread calls to awt methods:
   java.lang.Thread.State: BLOCKED (on object monitor)
        at java.awt.Component.setFont(java.desktop@11.0.19/Component.java:1932)
        - waiting to lock <0x00000000c1239508> (a java.awt.Component$AWTTreeLock)
        at java.awt.Container.setFont(java.desktop@11.0.19/Container.java:1777)
        at javax.swing.JComponent.setFont(java.desktop@11.0.19/JComponent.java:2769)
        at javax.swing.LookAndFeel.installColorsAndFont(java.desktop@11.0.19/LookAndFeel.java:209)
        at javax.swing.plaf.basic.BasicPanelUI.installDefaults(java.desktop@11.0.19/BasicPanelUI.java:77)
        at javax.swing.plaf.basic.BasicPanelUI.installUI(java.desktop@11.0.19/BasicPanelUI.java:62)
        at com.formdev.flatlaf.ui.FlatPanelUI.installUI(FlatPanelUI.java:69)
        at javax.swing.JComponent.setUI(java.desktop@11.0.19/JComponent.java:685)
        at javax.swing.JPanel.setUI(java.desktop@11.0.19/JPanel.java:150)
        at javax.swing.JPanel.updateUI(java.desktop@11.0.19/JPanel.java:126)
        at javax.swing.JPanel.<init>(java.desktop@11.0.19/JPanel.java:86)
        at javax.swing.JPanel.<init>(java.desktop@11.0.19/JPanel.java:95)
        at jalview.gui.Desktop.addProgressPanel(Desktop.java:2107)
        at jalview.gui.Desktop.startLoading(Desktop.java:2096)
        at jalview.io.FileLoader.run(FileLoader.java:302)
        at jalview.io.FileLoader._LoadFileWaitTillLoaded(FileLoader.java:233)
        at jalview.io.FileLoader.LoadFileWaitTillLoaded(FileLoader.java:186)
        at jalview.bin.Commands.processLinked(Commands.java:310)
        at jalview.bin.Commands.processArgs(Commands.java:131)
        at jalview.bin.Jalview.doMain(Jalview.java:841)

In this case, moving _LoadFileWaitTillLoaded to another thread that does not contend with any locks on the calling thread should help.