Adding module_utils/module_helper.py + big revamp in xfconf.py to use it (#1322)

* Big revamp in xfconf.py

- added plugin/module_utils/module_helper.py
  - scaffold class for writing modules, beyond standard AnsibleModule
  - automatic capture of exceptions
  - easier dependency testing
  - StateMixin to easily handle different behaviours for 'state' param
  - CmdMixin to easily run external commands
- adapted test_xfconf.py
  - the args for run_command are now lists instead of a string
  - value and previous_value were not being tested before (because xfconf wasn't filling results - see below)
  - added more tests: setting value to previous_value, getting non-existent property
- rewritten xfconf module, keeping the same results
  - original module posted results as ansible_facts, this version still does it for compatibility, but also adds to the module result

* Added suggestions from the PR

* Added russoz as maintainer for the module_utils/module_helper.py file

* Formatting using printf-style requires special treatment

Strings not containing substitution tokens must work as well.

* Tidied up variables in module definition

* Tests with ArgFormat and DependencyCtxMgr

* pytest parameters must be in the same order, it seems

* improved testing for the DependencyCtxMgr

* fixed test for older pythons

* Moved changed property to improve readability

* Added testcase for state: absent and adjusted xfconf after it

* Fixed param name environ_update in run_command()

* added changelog fragment

* fixed tests after run_command param change

.github/BOTMETA.yml

@@ -166,6 +166,9 @@ files:
   $module_utils/memset.py:
     maintainers: glitchcrab
     labels: cloud memset
+  $module_utils/module_helper.py:
+    maintainers: russoz
+    labels: module_helper
   $module_utils/net_tools/nios/api.py:
     maintainers: $team_networking sganesh-infoblox
     labels: infoblox networking