kcms: Add new Sound Theme KCM
Add a new KCM to select the global Sound Theme that will be used to play notifications.
The KCM lists the themes found in the system or user sound paths, the context information they provide, and several of the most used sounds to preview and compare.
The larger "play" button plays the example sound set by the theme, or a suitable alternative such as desktop-login (as many do not provide one)
This MR doesn't provide a "Get New Themes" button yet, but I'm looking into it:
- There is actually a "System Sounds" page in the store (resources are shared with the Gnome store as they are valid for both desktops), but it is currently hidden
- We also need to add that category (316) in the OCS API for KNewstuff
Another step forward towards #47