[applets/showdesktop] Fix errors due to uninitialized loader

Make loader synchronous, because there's no reason not to; and introduce
a new property to make the code safer and more self-descriptive.

    /usr/share/plasma/plasmoids/org.kde.plasma.showdesktop/contents/ui/main.qml:161: TypeError: Cannot read property 'active' of null
    /usr/share/plasma/plasmoids/org.kde.plasma.showdesktop/contents/ui/main.qml:39: TypeError: Cannot read property 'active' of null
    /usr/share/plasma/plasmoids/org.kde.plasma.showdesktop/contents/ui/main.qml:32: TypeError: Cannot read property 'active' of null