Sound panel: Select sound profile and customize system sounds

The original purpose for which I started working in this MR was to allow changing the tone from gnome-clocks (which made me wake up close to a heart attack every time...).

I found that to do that, a new profile had to be created. According to the Sound Theme Specification limited changes done by programs should go into a __custom profile, so I followed it (creating the profile if it did not exist). Then, in addition to the Alarm Clock, I added some basic sounds which are common to mobile systems.

In the end, I realized that part of this work is probably more appropriate to g-c-c (which already includes a very basic customization: Alert Sound). I will work on sending a MR to g-c-c, at least for the alarm clock and maybe also for the systems sounds. However, that is most likely going to take a while (C code + GNOME design guidelines + GNOME slow merging + ...) and I have no confidence that all the changes will make it upstream. So I thought it makes sense to first send the changes here and then work on getting them into g-c-c.