Fixes #30, added quick settings icon. (#32)

* Fixes #30, added quick settings icon.
* Added saving of enabled state, added some fixes and refactorings.
* Fixed a bug with turning on/off the VPN.
* Fixed possibility to add duplicate servers in DNS settings.

app/src/main/java/eu/neilalexander/yggdrasil/YggStateReceiver.kt

@@ -0,0 +1,53 @@
+package eu.neilalexander.yggdrasil
+
+import android.content.BroadcastReceiver
+import android.content.Context
+import android.content.Intent
+import android.content.IntentFilter
+import androidx.localbroadcastmanager.content.LocalBroadcastManager
+
+const val STATE_ENABLED = "enabled"
+const val STATE_DISABLED = "disabled"
+const val STATE_CONNECTED = "connected"
+const val STATE_RECONNECTING = "reconnecting"
+
+class YggStateReceiver(var receiver: StateReceiver): BroadcastReceiver() {
+
+    companion object {
+        const val YGG_STATE_INTENT = "eu.neilalexander.yggdrasil.YggStateReceiver.STATE"
+    }
+
+    override fun onReceive(context: Context?, intent: Intent?) {
+        if (context == null) return
+
+        val state = when (intent?.getStringExtra("state")) {
+            STATE_ENABLED -> State.Enabled
+            STATE_DISABLED -> State.Disabled
+            STATE_CONNECTED -> State.Connected
+            STATE_RECONNECTING -> State.Reconnecting
+            else -> State.Unknown
+        }
+        receiver.onStateChange(state)
+    }
+
+    fun register(context: Context) {
+        LocalBroadcastManager.getInstance(context).registerReceiver(
+            this, IntentFilter(YGG_STATE_INTENT)
+        )
+    }
+
+    fun unregister(context: Context) {
+        LocalBroadcastManager.getInstance(context).unregisterReceiver(this)
+    }
+
+    interface StateReceiver {
+        fun onStateChange(state: State)
+    }
+}
+
+/**
+ * A class-supporter with an Yggdrasil state
+ */
+enum class State {
+    Unknown, Disabled, Enabled, Connected, Reconnecting;
+}