Allow globally disabling imagery nag panels

[galen8183]

Add a configuration property to globally disable the imagery nag panel
("Imagery [...] might be misaligned."). Anyone frequently adding new imagery
layers should already know to check alignment. Therefore this is just an
annoyance for experienced users, and should be globally configurable. This can
be kept only in the advanced preferences so most users will still see the
warning.

My guess at the implementation:

src/org/openstreetmap/josm/gui/layer/AlignImageryPanel.java

@@ -87 +87 @@
      * @param infoToAdd ImageryInfo for which the nag panel should be created
      */
     public static void addNagPanelIfNeeded(ImageryInfo infoToAdd) {
+        BooleanProperty showAgainAny = new BooleanProperty("message.imagery.nagPanel", true);
+        if (!Boolean.TRUE.equals(showAgainAny.get()) {
+            return
+        }
+
         BooleanProperty showAgain = new BooleanProperty("message.imagery.nagPanel." + infoToAdd.getUrl(), true);
         MapFrame map = MainApplication.getMap();
         if (MainApplication.isDisplayingMapView() && Boolean.TRUE.equals(showAgain.get())