Use reliable and consistent method to warn of critical battery level

Currently at least two methods are used, resulting in two different dialogs:

• https://gitlab.xfce.org/xfce/xfce4-power-manager/-/blob/f6d5a34fa8a9c1125fc74f7bfb177bf78657add2/src/xfpm-manager.c#L323
• https://gitlab.xfce.org/xfce/xfce4-power-manager/-/blob/f6d5a34fa8a9c1125fc74f7bfb177bf78657add2/src/xfpm-power.c#L576

There should probably be only one.

Furthermore, in view of xfce4-session#97 (closed) or #164 (closed), it seems preferable to use a dialog specific to xfce4-power-manager, instead of or before invoking the xfce4-session dialog.