fix #256, add PRODID and VERSION to exported ics

app/src/main/kotlin/com/simplemobiletools/calendar/helpers/Constants.kt

@@ -74,6 +74,8 @@ val FLAG_ALL_DAY = 1
 // constants related to ICS file exporting / importing
 val BEGIN_CALENDAR = "BEGIN:VCALENDAR"
 val END_CALENDAR = "END:VCALENDAR"
+val CALENDAR_PRODID = "PRODID:-//Simple Mobile Tools//NONSGML Event Calendar//EN"
+val CALENDAR_VERSION = "VERSION:2.0"
 val BEGIN_EVENT = "BEGIN:VEVENT"
 val END_EVENT = "END:VEVENT"
 val BEGIN_ALARM = "BEGIN:VALARM"

app/src/main/kotlin/com/simplemobiletools/calendar/helpers/IcsExporter.kt

@@ -26,6 +26,8 @@ class IcsExporter {
 
             it.bufferedWriter().use { out ->
                 out.writeLn(BEGIN_CALENDAR)
+                out.writeLn(CALENDAR_PRODID)
+                out.writeLn(CALENDAR_VERSION)
                 for (event in events) {
                     out.writeLn(BEGIN_EVENT)
                     event.title.replace("\n", "\\n").let { if (it.isNotEmpty()) out.writeLn("$SUMMARY:$it") }