LibDesktop: Add menu_name() to AppFile

This really just takes the [App].Name config value and removes the
ampersands `&` from the name. These ampersands are hotkeys for the
system menu. Instead of typing them twice - error prone  - use the
fact that name is just menu_name without the ampersand. Remove the
ampersand to use as the name and use it as is as the menu_name.

Userland/Libraries/LibDesktop/AppFile.h

@@ -30,6 +30,7 @@ public:
     ByteString filename() const { return m_config->filename(); }
 
     ByteString name() const;
+    ByteString menu_name() const;
     ByteString executable() const;
     ByteString category() const;
     ByteString description() const;