Add support for groups of keys to SignEncryptWidget

There are not yet any groups in the model, but once there are
groups all keys in the selected groups are added to the list of
recipients.

GnuPG-bug-id: 5175, 5241
1 job for work/kloecker/group-support in 4 minutes (queued for 15 minutes and 32 seconds)
Status Job ID Name Coverage
  Build
failed #87409
build_clazy_clang_tidy

00:04:00

 
Name Stage Failure
failed
build_clazy_clang_tidy Build

-- Configuring incomplete, errors occurred!
See also "/builds/pim/kleopatra/build/CMakeFiles/CMakeOutput.log".
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Cleaning up file based variables
ERROR: Job failed: exit code 1