Removes more deprecated method usages

Change-Id: I6e592b5e5582ddb3ecc7294fc65c009f62e0e8c1
Reviewed-on: http://gerrit.dmdirc.com/1115
Reviewed-by: Chris Smith <chris@dmdirc.com>
Automatic-Compile: DMDirc Local Commits <dmdirc@googlemail.com>

src/com/dmdirc/addons/ui_swing/framemanager/tree/TreeFrameManager.java

@@ -287,8 +287,7 @@ public final class TreeFrameManager implements FrameManager,
 
                 for (FrameContainer window : rootWindows) {
                     addWindow(window);
-                    final FrameContainer[] childWindows = WindowManager.getChildren(
-                            window);
+                    final Collection<FrameContainer> childWindows = window.getChildren();
                     for (FrameContainer childWindow : childWindows) {
                         addWindow(window, childWindow);
                     }

src/com/dmdirc/addons/ui_swing/framemanager/windowmenu/FrameContainerMenuItem.java

@@ -121,7 +121,7 @@ public class FrameContainerMenuItem extends JMenuItem implements FrameInfoListen
     public void selectionChanged(final FrameContainer window) {
         if (frame.equals(window)) {
             setFont(getFont().deriveFont(Font.BOLD));
-            final FrameContainer parentWindow = WindowManager.getParent(window);
+            final FrameContainer parentWindow = window.getParent();
             if (parentWindow != null) {
                 manager.parentSelection(parentWindow);
             }

src/com/dmdirc/addons/ui_swing/framemanager/windowmenu/WindowMenuFrameManager.java

@@ -272,7 +272,7 @@ public final class WindowMenuFrameManager extends JMenu implements
     }
 
     private JMenu getParentMenu(final FrameContainer window) {
-        FrameContainer parent = WindowManager.getParent(window);
+        FrameContainer parent = window.getParent();
 
         if (parent == null) {
             return this;