core, citra_qt: Implement a save states file format and slot UI

10 slots are offered along with 'Save to Oldest Slot' and 'Load from Newest Slot'.

The savestate format is similar to the movie file format. It is called CST (Citra SavesTate), and is basically a 0x100 byte header (consisting of magic, revision, creation time and title ID) followed by Zstd compressed raw savestate data.

The savestate files are saved to the `states` folder in Citra's user folder. The files are named like `<Title ID>.<Slot ID>.cst`.

src/common/file_util.h

@@ -36,6 +36,7 @@ enum class UserPath {
     RootDir,
     SDMCDir,
     ShaderDir,
+    StatesDir,
     SysDataDir,
     UserDir,
 };