Improve how ImageMagick is included

[peteruithoven]

Currently we compile ImageMagick when doing flatpak packaging, this takes a very long time, making working on Flatpak related improvements frustrating.

I'd love to find a better way to include it, without needing to recompile it.

[peteruithoven]

Other apps seem to have a similar approach:

• https://github.com/muriloventuroso/pdftricks/blob/master/com.github.muriloventuroso.pdftricks.yml#L81-L92
• https://github.com/robertsanseries/ciano/blob/flatpak-packaging/com.github.robertsanseries.ciano.json#L36-L72