To be used to build the `Toolbelt Catalog`.
Fix https://issues.redhat.com/browse/QC-66

Just Tier:1 and only with 'local' provision.
We need to check fmf format works, not tmt features...

This will let fmf users to re-use the implementation for validating
structures that are not fmf nodes.

Signed-off-by: Cristian Le <cristian.le@mpsd.mpg.de>

Add support for filtering based on the node name by allowing plain
literals without the `key: name` format.

Expected usage with virtual tests while keeping the parent
test as well as hiding leaf nodes from selection

Fix: #221
Fix: #231

Replace `.pop()` calls with skipping "control" keys (when,
because...) during the merge.

Use case: Explicit set of the rules to be applied, regardless what
is in the Tree.

Add support for escaping boolean operators using the back slash
character `\` to allow for example regular expression grouping.

Co-authored-by: Cristian Le <cristian.le@mpsd.mpg.de>
Co-authored-by: Petr Šplíchal <psplicha@redhat.com>

People tend to try using parentheses in their filter expressions
for obvious reasons. But it is not possible. I feel like this
should be mentioned in the documentation. In fact, with DNF you
never need parentheses to express any expression and I think this
fact also needs to be noted there.

Signed-off-by: Ondrej Moris <omoris@redhat.com>
Co-authored-by: Petr Šplíchal <psplicha@redhat.com>

Let's document the more interesting features so that users are
aware of what's new in each release.