Add --old-metadata as a way to re-use previously generated icons and XML

The previous metadata must have been generated with --add-cache-id which does
increase the metadata size, but dramatically reduces the time to rebuild the
metadata for each tree.
Do not assume that paths in the package start at '.'
Support archives that don't include '/' as the first element

This fixes things for Arch packages, based on a patch by Fabien Bourigault.
Extract X-IBus-Symbol metadata from ibus components
Extract supported languages from ibus components
Parse the GObjectIntrospection GIR files to detect more GTK3 applications

Resolves https://github.com/hughsie/createrepo_as/issues/1
trivial: Partially revert the error prefixing

The logs get too wide...
Never add the 'other' language
trivial: Don't add empty X-IBus-Symbol metadata
Use as_util_get_possible_kudos()
Blacklist xscreensaver-*
Show AppData files installed but not used
hotot-qt requires the icon too
Do not include release text of package rebuilds

This uses a hacky heuristic, but seems to increase the SNR somewhat.
trivial: Default to creating 0.4.1 version metadata

This ensures we get captions for our screenshots.
Support the new metadata_license tag in AppData
Log when we try to overwrite existing metadata keys
Automatically remove font foundary names
trivial: Also log overwriting the Name and Summary values
Validate metadata in fonts to avoid writing junk