Undo-redo support on Test Plan tree modification

**Michel Nolard** ([Bug 42248](https://bz.apache.org/bugzilla//show_bug.cgi?id=42248&redirect=false)):
Hi !

I am very happy with JMeter, which is very useful and powerful ! So, I decided
to contribute in my way by pointing those little annoying things which the users
had to put up with but didn't notice or didn't take the time to report.

My thought are that a Undo/Redo feature is required while editing a test plan as
it is well known that user _do_ mistakes and don't want to be punished simply
for being human.

At first, I would argue that adding such a framework to JMeter would imply some
work, refactoring some things and implementing Command Pattern to ensure proper
efficiency and power. When I think a little bit more about it, I feel it to be
like an "internal XML patching system", as the problem is to apply modifications
to the plan test tree (which can be represented as an XML tree) and add more
modifications or revert some others.

I hope this won't be too hard to implement in JMeter. I simply don't know as
I've only _thought_ about it but not designed code to do it (which I am not able
to write at this time).

Votes in Bugzilla: 11
OS: All

Duplicates:
* https://github.com/apache/jmeter/issues/2502
* https://github.com/apache/jmeter/issues/2533

Depends on:
* https://github.com/apache/jmeter/issues/3442
* https://github.com/apache/jmeter/issues/3443
* https://github.com/apache/jmeter/issues/3444