GNOME SDK: Add blueprint-compiler
The blueprint-compiler is a command line utility that lets app developers write UI definitions using its own markup syntax. Many apps inside and outside GNOME Circle use it to avoid writing UIs in XML. The blueprint-compiler has been stable for quite a while now, and the project is currently applying for GNOME Circle. Given how popular the utility is, it might be worth it to add blueprint-compiler to the GNOME SDK on Flathub, so that apps no longer have to specify it manually as a build dependency.