missing <f:behavior> (convenience) tag

For converters/validators there is a convenience tag to integrate (simple)
custom implementations of these.

For behaviors that is not the case. A tag would nice; It should
"act" similar to its cousins, f:converter/validator
#### Environment
Operating System: All
Platform: All
#### Affected Versions
[2.1]

Start by reviewing how the existing f:converter and f:validator convenience tags are implemented, then compare that flow with behavior handling. The work is done when a corresponding f:behavior tag supports simple custom behavior implementations in the same way as its cousins; the issue does not name specific files or tests.

Written by the indexing model from the issue text.