feat: Customizable shortcuts/hotkeys (#439)

This implements the logic for allowing hotkeys with modifiers.
Implemented globalhotkeys and videocontrol hotkeys
Also implements saving the forward backwards seconds to the user.

Co-authored-by: PartyDonut <PartyDonut@users.noreply.github.com>

lib/models/settings/key_combinations.dart

@@ -0,0 +1,70 @@
+import 'dart:convert';
+
+import 'package:flutter/services.dart';
+
+import 'package:freezed_annotation/freezed_annotation.dart';
+
+import 'package:fladder/screens/settings/widgets/key_listener.dart';
+
+part 'key_combinations.freezed.dart';
+part 'key_combinations.g.dart';
+
+@Freezed(toJson: true, fromJson: true)
+class KeyCombination with _$KeyCombination {
+  const KeyCombination._();
+
+  factory KeyCombination({
+    @LogicalKeyboardSerializer() LogicalKeyboardKey? modifier,
+    @LogicalKeyboardSerializer() required LogicalKeyboardKey key,
+  }) = _KeyCombination;
+
+  factory KeyCombination.fromJson(Map<String, dynamic> json) => _$KeyCombinationFromJson(json);
+
+  @override
+  bool operator ==(covariant other) {
+    return other is KeyCombination && other.key.keyId == key.keyId && other.modifier?.keyId == modifier?.keyId;
+  }
+
+  @override
+  int get hashCode => key.hashCode ^ modifier.hashCode;
+
+  String get label => [modifier?.label, key.label].nonNulls.join(" + ");
+
+  static final Set<LogicalKeyboardKey> shiftKeys = {
+    LogicalKeyboardKey.shift,
+    LogicalKeyboardKey.shiftLeft,
+    LogicalKeyboardKey.shiftRight,
+  };
+
+  static final altKeys = {
+    LogicalKeyboardKey.alt,
+    LogicalKeyboardKey.altRight,
+    LogicalKeyboardKey.altLeft,
+  };
+
+  static final ctrlKeys = {
+    LogicalKeyboardKey.control,
+    LogicalKeyboardKey.controlLeft,
+    LogicalKeyboardKey.controlRight,
+  };
+
+  static final modifierKeys = {
+    ...shiftKeys,
+    ...altKeys,
+    ...ctrlKeys,
+  };
+}
+
+class LogicalKeyboardSerializer extends JsonConverter<LogicalKeyboardKey, String> {
+  const LogicalKeyboardSerializer();
+
+  @override
+  LogicalKeyboardKey fromJson(String json) {
+    return LogicalKeyboardKey.findKeyByKeyId(int.parse(jsonDecode(json))) ?? LogicalKeyboardKey.abort;
+  }
+
+  @override
+  String toJson(LogicalKeyboardKey object) {
+    return jsonEncode(object.keyId.toString());
+  }
+}