Add enemy speed as a room property

Currently, you can change platform speed, but not enemy speed, which is
always hardcoded to be 4. This commit fixes that, by adding the
"enemyv" property, which is an offset to the speed of 4. Since it
defaults to 0, older levels are not broken by this change.

desktop_version/lang/ja/strings.xml

@@ -658,6 +658,7 @@ Steam Deckには対応していません。" explanation="" max="38*5" max_local
     <string english="Tileset Colour Changed" translation="タイルセットのカラーを変更しました" explanation="level editor, user changed the tileset colour/variant of the room" max="38*3" max_local="38*2"/>
     <string english="Enemy Type Changed" translation="敵の種類を変更しました" explanation="level editor, user changed enemy appearance for the room" max="38*3" max_local="38*2"/>
     <string english="Platform speed is now {speed}" translation="プラットフォームの速度を {speed} に変更しました" explanation="level editor, user changed speed of platforms for the room" max="38*3" max_local="38*2"/>
+    <string english="Enemy speed is now {speed}" translation="" explanation="level editor, user changed speed of enemies for the room" max="38*3" max_local="38*2"/>
     <string english="Reloaded resources" translation="リソースを再読み込みしました" explanation="level editor, reloaded graphics assets/resources, music and sound effects" max="38*3" max_local="38*2"/>
     <string english="ERROR: Invalid format" translation="エラー: 無効な記述形式" explanation="user was supposed to enter something like `12,12`, but entered `as@df`" max="38*3" max_local="38*2"/>
     <string english="Loaded map: {filename}.vvvvvv" translation="{filename}.vvvvvv をロードしました" explanation="successfully loaded level file" max="38*3" max_local="38*2"/>