Add option to check file age

Services.ZUGFeRDService/Config.vb

@@ -37,6 +37,13 @@
         ''' </summary>
         Public Property MaxAttachmentSizeInMegaBytes As Integer = -1
 
+        ''' <summary>
+        ''' Minimum file age. If one file is newer than this, the whole group will be skipped
+        ''' to ensure that all files of a group have been created.
+        ''' </summary>
+        ''' <returns></returns>
+        Public Property MinFileAgeInMinutes As Integer = 5
+
         Public Property AllowFacturX As Boolean = False
         Public Property AllowXRechnung As Boolean = False
         Public Property AllowZugferd10 As Boolean = True