Add migration ability.

Various bug fixes and code cleanup.

app/src/main/java/exh/metadata/MetadataHelper.kt

@@ -9,15 +9,22 @@ class MetadataHelper {
             = exGalleryBook().write(galleryMetadata.galleryUniqueIdentifier(), galleryMetadata)
 
     fun fetchMetadata(url: String, exh: Boolean): ExGalleryMetadata?
-            = ExGalleryMetadata().apply {
-        this.url = url
-        this.exh = exh
-        return exGalleryBook().read<ExGalleryMetadata>(galleryUniqueIdentifier())
+            = ExGalleryMetadata().let {
+        it.url = url
+        it.exh = exh
+        return exGalleryBook().read<ExGalleryMetadata>(it.galleryUniqueIdentifier())
     }
 
     fun getAllGalleries() = exGalleryBook().allKeys.map {
         exGalleryBook().read<ExGalleryMetadata>(it)
     }
 
+    fun hasMetadata(url: String, exh: Boolean): Boolean
+            = ExGalleryMetadata().let {
+        it.url = url
+        it.exh = exh
+        return exGalleryBook().exist(it.galleryUniqueIdentifier())
+    }
+
     fun exGalleryBook() = Paper.book("gallery-ex")!!
 }