ui.rc files: consistently use <gui> instead of deprecated <kpartgui>

ui/kgetui.rc

-<!DOCTYPE kpartgui>
-<kpartgui name="kget" version="8">
+<?xml version="1.0"?>
+<!DOCTYPE gui SYSTEM "kpartgui.dtd">
+<gui name="kget" version="8">
 <MenuBar>
     <Menu noMerge="1" name="file"><text>&amp;File</text>
         <Action name="new_download"/>
@@ -44,4 +45,4 @@
     <Action name="delete_selected_download"/>
     <Action name="delete_all_finished"/>
 </ToolBar>
-</kpartgui>
+</gui>