Support filtering for the absence of a label

[GBognar]

Support filtering for the absence of a label

The situations where the need for this feature arise

To handle certain cases, either the excessive use of labels is needed or the excessive use of new columns.

• Example 1: A task might have a label like Waiting for 3rd party. For finding tasks that can not be handled
  right now, the label is a good solution. But to be able to filter for tasks that can be handled right now, either a
  separate label should be created (e.g. No 3rd party involved) or an additional column (e.g. Tasks waiting for 3rd party). With both solutions additional activity is needed, while filtering for the absence of a label would
  be enough.
• Example 2: When a task have multiple labels some of which are orthogonal to each other, searching for
  a task (with a combination of the mentioned labels) is either impossible or extremely cumbersome.

A requested solution

Please support filtering for the abence of a label.

Currently filtering is supported only for the presence of a label, i.e. a label is either ticked or not in the search dialogue,
'not ticked' being represented by a lighter tick.The absent (i.e. should not be present) state could be supported by adding a third state for the search dialogue as 'definitely not ticked' (i.e. crossed out), represented by a crossed tick or a cross with the
same intensity as the 'selected tick'.

Additional context

This would change the search logic for labels from the current, two-valued present/does not matter to a three-valued
present/does not matter/absent version.

[Spartachetto]

I guess this is related to nextcloud/server#4851

[GBognar]

It is, but much simpler due to its smaller scope.

The only addition is the negation (NOT), so there are no additional AND/OR operations and so there is no additional need to support operator precedence and/or grouping/parenthesis.

[stale]

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