Load custom Qt themes from yuzu data directory

- Directory is qt_themes, each theme must be in one folder
    - It should contain a file "style.qss"
    - It may contain an "icons" sub-directory, to override included icons
      (with files like mytheme/icons/colorful/48x48/star.png for example)
    - Directories ending by "_dark" are reserved for dark variant icons.
      They are not listed as themes in the UI.
- If theme directory contains "dark" or "midnight", theme will be considered dark

src/common/fs/path_util.h

@@ -17,6 +17,7 @@ enum class SuyuPath {
     ConfigDir,      // Where config files are stored.
     CrashDumpsDir,  // Where crash dumps are stored.
     DumpDir,        // Where dumped data is stored.
+    IconsDir,       // Where Icons for Windows shortcuts are stored.
     KeysDir,        // Where key files are stored.
     LoadDir,        // Where cheat/mod files are stored.
     LogDir,         // Where log files are stored.
@@ -26,7 +27,7 @@ enum class SuyuPath {
     SDMCDir,        // Where the emulated SDMC is stored.
     ShaderDir,      // Where shaders are stored.
     TASDir,         // Where TAS scripts are stored.
-    IconsDir,       // Where Icons for Windows shortcuts are stored.
+    ThemesDir,      // Where users should put their custom themes
 };
 
 /**