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/widgets/settingspages/GeneralPage.cpp

@@ -114,8 +114,18 @@ void GeneralPage::initLayout(GeneralPageView &layout)
     auto &s = *getSettings();
 
     layout.addTitle("Interface");
-    layout.addDropdown("Theme", {"White", "Light", "Dark", "Black"},
-                       getApp()->themes->themeName);
+
+    layout.addDropdown<QString>(
+        "Theme", getApp()->themes->availableThemes(),
+        getApp()->themes->themeName,
+        [](const auto *combo, const auto &themeKey) {
+            return combo->findData(themeKey, Qt::UserRole);
+        },
+        [](const auto &args) {
+            return args.combobox->itemData(args.index, Qt::UserRole).toString();
+        },
+        {}, Theme::fallbackTheme.name);
+
     layout.addDropdown<QString>(
         "Font", {"Segoe UI", "Arial", "Choose..."},
         getApp()->fonts->chatFontFamily,