fix: fix system icons

This sets the fallback icon theme to the current(system default) icon theme before
launcher specific themes are applied

And removes `Inherits` line of multimc/legacy icon theme because it can end up making it
inherit a default theme set from /usr/share/icons/default/index.theme
instead of the user configured theme (probably a qt bug?)

Signed-off-by: leia uwu <leia@tutamail.com>

launcher/ui/themes/ThemeManager.cpp

@@ -36,6 +36,7 @@
 
 ThemeManager::ThemeManager()
 {
+    QIcon::setFallbackThemeName(QIcon::themeName());
     themeDebugLog() << "Determining System Widget Theme...";
     const auto& style = QApplication::style();
     m_defaultStyle = style->objectName();