Make sprite and widget sprite overrides more flexible

- Expose NodeCache interface in RuneLite
- Expose Client.getWidgetSpriteCache and reset method on it
- Change setters for sprite and widget overrides to be getters

This allows for more than 1 plugin to use the widget/sprite override API
in future.

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

runescape-api/src/main/java/net/runelite/rs/api/RSClient.java

@@ -645,6 +645,7 @@ public interface RSClient extends RSGameEngine, Client
 	void setCompass(SpritePixels spritePixels);
 
 	@Import("widgetSpriteCache")
+	@Override
 	RSNodeCache getWidgetSpriteCache();
 
 	@Import("oculusOrbState")

runescape-api/src/main/java/net/runelite/rs/api/RSNodeCache.java

@@ -24,9 +24,10 @@
  */
 package net.runelite.rs.api;
 
+import net.runelite.api.NodeCache;
 import net.runelite.mapping.Import;
 
-public interface RSNodeCache
+public interface RSNodeCache extends NodeCache
 {
 	@Import("get")
 	RSCacheableNode get(long id);