MusicSheet: add setter for TitleString, SubtitleString, ComposerString, LyricistString (no need to give Label)

with setters for Title etc, the user would need to create a new Label() and thus import that class.
these setters create the label for you.

src/MusicalScore/MusicSheet.ts

@@ -164,6 +164,9 @@ export class MusicSheet /*implements ISettableMusicSheet, IComparable<MusicSheet
             return "";
         }
     }
+    public set TitleString(value: string) {
+        this.Title = new Label(value);
+    }
     public get SubtitleString(): string {
         if (this.subtitle) {
             return this.subtitle.text;
@@ -171,6 +174,9 @@ export class MusicSheet /*implements ISettableMusicSheet, IComparable<MusicSheet
             return "";
         }
     }
+    public set SubtitleString(value: string) {
+        this.Subtitle = new Label(value);
+    }
     public get ComposerString(): string {
         if (this.composer) {
             return this.composer.text;
@@ -178,6 +184,9 @@ export class MusicSheet /*implements ISettableMusicSheet, IComparable<MusicSheet
             return "";
         }
     }
+    public set ComposerString(value: string) {
+        this.Composer = new Label(value);
+    }
     public get LyricistString(): string {
         if (this.lyricist) {
             return this.lyricist.text;
@@ -185,6 +194,9 @@ export class MusicSheet /*implements ISettableMusicSheet, IComparable<MusicSheet
             return "";
         }
     }
+    public set LyricistString(value: string) {
+        this.Lyricist = new Label(value);
+    }
     public get Title(): Label {
         return this.title;
     }