snapcraft does not allow dynamically generated metadata

[Björn Michaelsen]

Snapcrafts metadata is expected to be static, which might be ok for smaller packages, but bigger software will often dynamically generate even metadata like desktop files.

E.g.:

> https://git.launchpad.net/~bjoern-michaelsen/df-libreoffice/+git/libreoffice-snap-playground/tree/setup/gui?id=29980f3f79ea804fd32cf88f07b8eecfbb54ae96

are currently static dumps of files generated by an upstream build. (Upstream source: https://gerrit.libreoffice.org/gitweb?p=core.git;a=tree;f=sysui/desktop/menus;h=23368a604813950e5cb78a0574320c5333a3f1d4;hb=584262fed1368dba382d88dfd25fe35ac7d4b349)

Upstream updates to the desktop files will either be ignored currently, or changes in the snapcraft package will be mangled over by refreshing/regenerating from upstream (e.g. for updates in localization).

So it would be great to allow snap package metadata be dynamically generated by the build (or have a best practice for that).