[Feature] Allow using CharSequence for popup text.

Fixes: #52

Author: zhanghai

library/src/main/java/me/zhanghai/android/fastscroll/FastScroller.java

@@ -195,7 +195,7 @@ private void onPreDraw() {
         layoutView(mThumbView, thumbLeft, thumbTop, thumbLeft + mThumbWidth,
                 thumbTop + mThumbHeight);
 
-        String popupText = mViewHelper.getPopupText();
+        CharSequence popupText = mViewHelper.getPopupText();
         boolean hasPopup = !TextUtils.isEmpty(popupText);
         mPopupView.setVisibility(hasPopup ? View.VISIBLE : View.INVISIBLE);
         if (hasPopup) {
@@ -443,7 +443,7 @@ public interface ViewHelper {
         void scrollTo(int offset);
 
         @Nullable
-        default String getPopupText() {
+        default CharSequence getPopupText() {
             return null;
         }
     }

library/src/main/java/me/zhanghai/android/fastscroll/PopupTextProvider.java

@@ -21,5 +21,5 @@
 public interface PopupTextProvider {
 
     @NonNull
-    String getPopupText(int position);
+    CharSequence getPopupText(int position);
 }

library/src/main/java/me/zhanghai/android/fastscroll/RecyclerViewHelper.java

@@ -118,7 +118,7 @@ public void scrollTo(int offset) {
 
     @Nullable
     @Override
-    public String getPopupText() {
+    public CharSequence getPopupText() {
         PopupTextProvider popupTextProvider = mPopupTextProvider;
         if (popupTextProvider == null) {
             RecyclerView.Adapter<?> adapter = mView.getAdapter();

sample/src/main/java/me/zhanghai/android/fastscroll/sample/LocaleListAdapter.java

@@ -80,7 +80,7 @@ public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
 
     @NonNull
     @Override
-    public String getPopupText(int position) {
+    public CharSequence getPopupText(int position) {
         Locale locale = getItem(position);
         return locale.toString().substring(0, 1).toUpperCase(Locale.getDefault());
     }