Move InitSystrayPanel to Systraybar::InitPanel.
Move InitBatteryPanel to Battery::InitPanel.
Move InitTaskbarPanel to Panel::InitTaskbar.
Move InitLauncherPanel to Launcher::InitPanel.
Use the std namespace for <ctime> functions.
On a second thought, let's not reinvent the wheel.

We don't need to recreate std::unique_ptr<>, as it already accepts a custom deleter.
Add the util::GObjectPtr wrapper.