Rework the HTTP Header Manager

**Michael Osipov** ([Bug 62279](https://bz.apache.org/bugzilla//show_bug.cgi?id=62279&redirect=false)):
In the quest of BZ 62252 and BZ 62261 it turned out that the current HTTP Header Manager has some shortcomings.

* It is not possible to remove a header from being added via parent/child configuration
* It is not possible to have multiple header values.

We need basically to add a third column in the pane with the label "Action". An enum with "Add", "Replace" and "Remove".

"Add": Adds a header value even is already present in the parent or in the child. Multiple header values are valid according to RFC.
"Replace": Replaces the (all) previous header value(s) with this value.
"Remove": Removes one distinct matching header value or if left blank, all values.

There might be more cases.

See also:
* https://github.com/apache/jmeter/issues/5998

OS: All