This makes the manifest use the SDK's master branch, updates the build
rules, and explicits to build Flips' master branch.
Signed-off-by: Adrien Plazas <aplazas@gnome.org>
This allows to install Flips on Linux desktops, with all the modern
bells and whistles, like SVG icons, AppStream metadata, a flatpak
manifest, and the com.github.Alcaro.Flips reverse domain name app ID.