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/storage/region_store.dart

@@ -0,0 +1,53 @@
+import 'package:meshcore_open/storage/channel_region_store.dart';
+import 'package:meshcore_open/storage/channel_store.dart';
+
+import 'prefs_manager.dart';
+
+typedef Region = String;
+
+class RegionStore {
+  static const String key = 'regions';
+  String publicKeyHex = '';
+  set setPublicKeyHex(String value) =>
+      publicKeyHex = value.length >= 10 ? value.substring(0, 10) : '';
+
+  List<Region> loadRegions() {
+    final prefs = PrefsManager.instance;
+    List<Region>? region = prefs.getStringList(key);
+    return region ?? [];
+  }
+
+  void saveRegions(List<Region> regions) {
+    final prefs = PrefsManager.instance;
+
+    var distinctRegions = [
+      ...{...regions},
+    ];
+
+    distinctRegions.sort();
+    prefs.setStringList(key, distinctRegions);
+  }
+
+  void addRegion(Region region) {
+    final regions = loadRegions();
+    regions.add(region);
+    saveRegions(regions);
+  }
+
+  Future<void> removeRegion(Region region) async {
+    final regions = loadRegions();
+    final channelStore = ChannelStore();
+    final channelRegionStore = ChannelRegionStore();
+    channelStore.setPublicKeyHex = publicKeyHex;
+    channelRegionStore.setPublicKeyHex = publicKeyHex;
+
+    for (var channel in await channelStore.loadChannels()) {
+      var channelRegion = await channelRegionStore.loadRegion(channel.index);
+      if (channelRegion == region) {
+        channelRegionStore.saveRegion(channel.index, '');
+      }
+    }
+    regions.remove(region);
+    saveRegions(regions);
+  }
+}