flatpak: Update the manifest

[Kekun]

This makes the manifest use the SDK's master branch, updates the build rules, and explicits to build Flips' master branch.

[Alcaro]

That removes the need to update that thing at regular intervals, which is my main objection to that file. Looks good to me.

No clue what that -DFLATPAK is supposed to do, nothing in Flips reads that (there is a slight behavior difference under Flatpak, but it's a runtime check). But I guess it's harmless.

[Kekun]

Regarding -DFLATPAK I initially added it in the Flatpak manifest, and you removed it in 6ae40f0. It's indeed probably useless, but because https://github.com/flathub/com.github.Alcaro.Flips/blob/master/com.github.Alcaro.Flips.json was left intact and included it, I added it back without thinking nor investigating. My mistake. I probably cargoculted it from somewhere else 4 years ago.