Fix client-sided field initialisers being used in common classes

Fixes #231. Fabric Loader 0.15 fixes the treatment of client-only fields,
so this code crashed before.

Author: Juuxel

src/main/java/io/github/cottonmc/cotton/gui/widget/WItemSlot.java

@@ -70,7 +70,7 @@ public class WItemSlot extends WWidget {
 	private final List<ValidatedSlot> peers = new ArrayList<>();
 	@Nullable
 	@Environment(EnvType.CLIENT)
-	private BackgroundPainter backgroundPainter = null;
+	private BackgroundPainter backgroundPainter;
 	@Nullable
 	private Icon icon = null;
 	private Inventory inventory;

src/main/java/io/github/cottonmc/cotton/gui/widget/WPanel.java

@@ -26,7 +26,7 @@ public abstract class WPanel extends WWidget {
 	 */
 	protected final List<WWidget> children = new WidgetList(this, new ArrayList<>());
 	@Environment(EnvType.CLIENT)
-	private BackgroundPainter backgroundPainter = null;
+	private BackgroundPainter backgroundPainter;
 
 	/**
 	 * Removes the widget from this panel.

src/main/java/io/github/cottonmc/cotton/gui/widget/WSlider.java

@@ -25,7 +25,7 @@ public class WSlider extends WAbstractSlider {
 
 	@Environment(EnvType.CLIENT)
 	@Nullable
-	private BackgroundPainter backgroundPainter = null;
+	private BackgroundPainter backgroundPainter;
 
 	public WSlider(int min, int max, Axis axis) {
 		super(min, max, axis);