Platform preferences

Update: we decided to radically simplify the design and only have one page.

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Heavily WIP:

 * [x] General:
   * [x] Use `GSettings` to store cores
 * [ ] Platforms:
   * [x] Add a platform register
   * [ ] Implement platform state
 * [ ] Add platform pages to preferences UI:
   * [x] Add custom sidebar to preferences (shared with !155, updated to support separators)
   * [x] Add dummy platform page
   * [x] Populate sidebar with platform pages
   * [x] Remove Plugins page
 * [ ] Libretro:
   * [x] Add a `RetroPlatform`
    * [x] List cores
    * [ ] List firmwares
   * [x] Add a way to choose core
   * [ ] Add a firmware installer
   * [ ] Add generic libretro page. Plugins should be able to reuse cores and firmware part, but add their own preferences
 * [ ] Plugins:
   * [x] ~~Add a way for plugins to use `GSettings`~~ — !214 has an example
   * [ ] Add desktop page
   * [ ] Add Steam page (???)
   * [ ] Add standalone libretro page (???)
   * [ ] ~~Add LÖVE page (???)

Fixes, but not quite, #22 (closed)