Turn resources (UI files, pixmaps) into a GResource

Added siril_resource.xml

Also adjust the loaders to load UI, shortcuts, pixmaps and CSS from resources.

No longer install UI files or unnecessary pixmaps to the user's filesystem.

TODO: the last thing for this MR is we should add xmllint to the Windows crossbuild environment (need to check, possibly also the native one?), as it is used to strip out unneeded whitespace and comments from the UI files to make them smaller. Everything will still work without it, but the whitespace removal will be stripped making for a slightly larger executable than necessary.