Provide an opt-in mechanism for applications to load a different base theme

As discussed in gtk#4230 (closed), it seems that libadwaita is open to core GNOME applications that are intended to run on different distributions to set a flag indicating they are willing to load the system theme. The implementation details of this, I am not sure. libadwaita loads the Adwaita theme from a resource. It could either load a theme from another setting, or re-use the gtk-theme-name setting if the application flags that it supports it.