Add PluginMetadata::UnsetGroup()

The postcondition is that GetGroup returns nullopt.

include/loot/metadata/plugin_metadata.h

@@ -180,6 +180,11 @@ public:
    */
   LOOT_API void SetGroup(const std::string& group);
 
+  /**
+   * Unsets the plugin's group.
+   */
+  LOOT_API void UnsetGroup();
+
   /**
    * Set the files that the plugin must load after.
    * @param after

src/api/metadata/plugin_metadata.cpp

@@ -220,6 +220,10 @@ void PluginMetadata::SetGroup(const std::string& group) {
   group_ = group;
 }
 
+void PluginMetadata::UnsetGroup() {
+  group_ = std::nullopt;
+}
+
 void PluginMetadata::SetLoadAfterFiles(const std::set<File>& l) {
   loadAfter_ = l;
 }

src/tests/api/internals/metadata/plugin_metadata_test.h

@@ -476,6 +476,17 @@ TEST_P(PluginMetadataTest, simpleMessagesShouldReturnMessagesAsSimpleMessages) {
   EXPECT_EQ("content3", simpleMessages.back().text);
 }
 
+TEST_P(PluginMetadataTest, unsetGroupShouldLeaveNoGroupValueSet) {
+  PluginMetadata plugin;
+  EXPECT_FALSE(plugin.GetGroup().has_value());
+
+  plugin.SetGroup("test");
+  EXPECT_EQ("test", plugin.GetGroup().value());
+
+  plugin.UnsetGroup();
+  EXPECT_FALSE(plugin.GetGroup().has_value());
+}
+
 TEST_P(PluginMetadataTest,
        hasNameOnlyShouldBeTrueForADefaultConstructedPluginMetadataObject) {
   PluginMetadata plugin;