Make the "add new" menu button a hamburger menu

I know hamburger menus sometimes aren't amazing, but the current icon is
misleading. It's a plus button which generally means "create new".
However the menu is full of actions not related to creating new things,
including:

- Explore Rooms
- Find your Friends
- Scan a QR Code

These actions may technically result in a new room appearing in the
sidebar, but that's not a user's definition of creating a new thing;
these are *joining* a thing, and the fact that a new entry appears in
the sidebar is an implementation detail.

As a result the existing icon is inaccurate, and also holds back the
menu from adding additional items in the future that are even less
related to creating new rooms. An example would be the quick room
switcher, which is not exposed visibly in the UI anywhere, and could not
logically live in the current menu without changing its icon and text.

src/qml/ExploreComponent.qml

@@ -100,8 +100,8 @@ RowLayout {
         display: QQC2.AbstractButton.IconOnly
         checkable: true
         action: Kirigami.Action {
-            text: i18n("Create rooms and chats")
-            icon.name: "irc-join-channel"
+            text: i18nc("@action:button", "Show Menu")
+            icon.name: "application-menu-symbolic"
             onTriggered: {
                 if (Kirigami.isMobile) {
                     const menu = mobileMenu.createObject();