Add identity management

Manage several TeamSpeak identities like the official client instead of
using one auto-generated identity file.

- IdentityStore/IdentityEntry keep each identity as a TeamSpeak-format INI
  in ~/.ts3jclient/identities, so files interchange with the TS3 client;
  supports create, import, export, rename, remove and a cancellable
  security-level search. A legacy identity.ini is migrated in on first load.
- Settings hold the default identity; bookmarks may pin their own, falling
  back to the default when unset.
- TeamspeakConnection now connects with the identity it is handed.
- Swing: Tools -> Identities manager, plus an identity drop-down in the
  connect and bookmark forms.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

ts3-client/swing/src/main/java/com/ts3client/ui/IdentityChooser.java

@@ -0,0 +1,48 @@
+package com.ts3client.ui;
+
+import com.ts3client.config.IdentityEntry;
+import com.ts3client.config.IdentityStore;
+
+import javax.swing.DefaultListCellRenderer;
+import javax.swing.JComboBox;
+import javax.swing.JList;
+import java.awt.Component;
+
+/**
+ * Drop-down for picking the identity to connect with. Optionally offers a
+ * "use the default identity" entry for places where the choice may stay unset
+ * (bookmarks), so changing the default later applies retroactively.
+ */
+final class IdentityChooser extends JComboBox<Object> {
+
+    private static final String USE_DEFAULT = "‹Default identity›";
+
+    IdentityChooser(IdentityStore identities, boolean allowDefault, String selectedId) {
+        if (allowDefault) addItem(USE_DEFAULT);
+        for (IdentityEntry e : identities.all()) addItem(e);
+
+        IdentityEntry selected = identities.byId(selectedId);
+        if (selected != null) {
+            setSelectedItem(selected);
+        } else if (allowDefault) {
+            setSelectedItem(USE_DEFAULT);
+        }
+
+        setRenderer(new DefaultListCellRenderer() {
+            @Override
+            public Component getListCellRendererComponent(JList<?> list, Object value, int index,
+                                                          boolean isSelected, boolean cellHasFocus) {
+                String text = value instanceof IdentityEntry
+                        ? ((IdentityEntry) value).getName()
+                        : String.valueOf(value);
+                return super.getListCellRendererComponent(list, text, index, isSelected, cellHasFocus);
+            }
+        });
+    }
+
+    /** Selected identity id, or an empty string when the default is to be used. */
+    String getSelectedIdentityId() {
+        Object v = getSelectedItem();
+        return v instanceof IdentityEntry ? ((IdentityEntry) v).getId() : "";
+    }
+}