Support for automatic dark/light theming

[idoric]

In the same way that it’s possible to automatically adjust night/day mode (at sunrise/sunset times, possibly based one’s location), I suggest adding the ability to automatically adjust dark/light theme.

This proposal is different from #202, which suggested the use of a specific external tool (darkman). The idea here would be to combine two existing dms capabilities: trigger an action at sunset/sunrise, and switch between dark and light theme, with the adapted GUI.

A possible advantage over darkman is that “darkman is not designed to be used interactively: it's designed to be set up once, and run in the background” (according to its presentation). However, it may be useful to temporarily disable the night mode and the dark theme (personally, I need it because of my bank's security system for online payments, with a device that needs to read a kind of QR code in color, and which is very sensitive to global brightness and colorimetry), and being able to do it directly from the dms’s GUI would be simpler.

[bbedward]

For gamma, we rely on an external tool too it's not built-in. It would be gammastep + (optionally) geoclue for location.

This type of stuff is really annoying in QML, but now that dms is not just quickshell (quickshell+GO) I'd be open to enriching more functionality in the backend side of things (including, maybe replacing gammastep all together, also).