Support opening folders with media files

D&D folder with videos onto Clapper window to play them as video playlist. If folder contains exactly one video and subtitle file, then that video will be played with subtitles automatically applied.

src/app.js

@@ -38,8 +38,7 @@ class ClapperApp extends AppBase
     {
         super.vfunc_open(files, hint);
 
-        this._openFiles(files);
-        this.activate();
+        this._openFilesAsync(files).then(() => this.activate()).catch(debug);
     }
 
     _onWindowMap(window)