Summary:
Source formatting is a feature seemingly only used by some.
So it makes sense to allow disabling this feature, for some minimal smaller
runtime footprint and less unused clutter in the UI (like menu actions or
settings).

This patch achieves this to a good degree:
* makes the plugins astyle & customscript normal global plugins which can be
  enabled/disabled by the user
* removes the SourceFormatterController actions from the menus when there are
  no formatters available
* shows no source formatting settings page in the project settings
  when there are no formatters available

Not yet done is to hide the source formatting settings page from the
application settings dialog in case no formatters are available. That might
need some more custom logic all over the shell code, which is not so nice.
Instead the option should be investigated to make the source formatting
controller a normal plugin with a suited interface, which then can be
queried by other code needing that service.

Test Plan:
See plugins astyle & customscript turn up in plugin selection settings.
Disable and enable both and see how actions in the main Edit menu and the
context menu on file items are present or not present, as well as the
formatting settings page in the project settings dialog being shown or not.

See also the formatting settings pages for projects and the global one only
offering the formatters of the enabled plugins.

Reviewers: #kdevelop, kfunk, mwolff

Reviewed By: #kdevelop, kfunk, mwolff

Subscribers: mwolff, kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D8492