Add region management

This adds region management: the user can manage a list of available regions
and for each channel pick a region from that list to apply to messages.

Region discovery from nearby repeaters will be done in a separate PR.

This is a part of the work needed for #120.

lib/models/channel.dart

@@ -24,6 +24,10 @@ class Channel {
 
   bool get isPublicChannel => pskHex == publicChannelPsk;
 
+  bool get isHashtagChannel => name.startsWith('#');
+
+  bool get isPrivateChannel => !isPublicChannel && !isHashtagChannel;
+
   static Channel? fromFrame(Uint8List frame) {
     // CHANNEL_INFO format:
     // [0] = RESP_CODE_CHANNEL_INFO (18)