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Fix some wrong build options for aarch64; correct patch pathes

Also (re-)adds some debian armhf-patches – not really needed, but making
it easier to apply the libstdc++-related patches without having to
modify them.

The AppImage validation tools complain even when one follows their
recommendations on supplying AppStream metadata and with correctly named
files. I'm now starting to understand why most AppImages don't bother
with it.

Also installs some tools for appstream validation, which unfortunately
show that currently the appstream file used has some issues.

Remove plugin certificates and some pre-installed extensions

Closes #59 and #112

See merge request !8

Via `sed` instead of a patch, in the hope that it might be less of a
maintenance burden compared to patch witch changes in the file to patch
with subsequent upstream releases.

Should only be merged once it's ensured there are no unforseen side
effects when built with the Plugin Certificates removed.