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.hpp

@@ -37,6 +37,9 @@ public:
     // Plugin files live here. <appDataDirectory>/Plugins
     QString pluginsDirectory;
 
+    // Custom themes live here. <appDataDirectory>/Themes
+    QString themesDirectory;
+
     bool createFolder(const QString &folderPath);
     bool isPortable();