lib: restructure to make space for database APIs

lib/models/jmdict/word_search_result.dart

@@ -0,0 +1,3 @@
+class WordSearchResult {
+  // TODO: implement me
+}

lib/models/kanjidic/kanji_search_result.dart

@@ -0,0 +1,3 @@
+class KanjiSearchResult {
+  // TODO: implement me
+}

lib/models/radkfile/radicals_search_result.dart

@@ -0,0 +1,3 @@
+class RadicalsSearchResult {
+  // TODO: implement me
+}