Fix caching bug causing some strings to be lost (closes #113)

getTag() would cache the result, but after setting a string
to empty content, the cached result wouldn't be invalidated.

This caused some operations to update the contents of a now
ghost tag, hence losing the changes when updating the cache.

src/app/src/main/java/io/github/lonamiwebs/stringlate/classes/resources/Resources.java

@@ -218,6 +218,8 @@ void loadTag(ResTag rt) {
 
     public void deleteId(String resourceId) {
         mStrings.remove(resourceId);
+        if (mLastTag != null && mLastTag.getId().equals(resourceId))
+            mLastTag = null;
     }
 
     //endregion