Add the ability to select custom themes in the settings dialog (#4570)

Themes are loaded from the Themes directory (under the Chatterino directory, so %APPDATA%/Chatterino2/Themes).

Themes are json files (see the built in themes as an example).

After importing a theme, you must restart Chatterino for it to show up in the settings

src/singletons/Paths.cpp

@@ -142,6 +142,7 @@ void Paths::initSubDirectories()
     this->miscDirectory = makePath("Misc");
     this->twitchProfileAvatars = makePath("ProfileAvatars");
     this->pluginsDirectory = makePath("Plugins");
+    this->themesDirectory = makePath("Themes");
     this->crashdumpDirectory = makePath("Crashes");
     //QDir().mkdir(this->twitchProfileAvatars + "/twitch");
 }