feat: add confirmation for resetting panel settings

[Zij-IT]

I accidentally reset my settings, and was surprised by the lack of a confirmation for the action. This PR would address #1967. I made the assumption that this being moved from libcosmic to cosmic-settings was an indication that this is something that was worth a PR.

This PR adds a confirmation dialog for panel settings after clicking on "Reset to default". The solution is based on the dialog found in the WiFi (pages/networking/wifi.rs).

This only implements the fix for the panel-settings. AFAICT this could also be applied to wallpaper, appearance and shortcuts, which all three use a simple "Reset to default" without confirmation. If this gets accepted, and a similar implementation is desired for the other locations, just let me know.

P.S.: I ran just check in cosmic-settings which resulted in about 252 unrelated warnings.

Edit: Force push was because I decided to have all the .dialog changes in the same match instead of splitting between the top and bottom matches.

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