Instead of implicitly disabling plugins if their dependencies are
disabled, implicitly enable plugins if some enabled plugin needs it.
That will avoid issues if people disable plugins (e.g. QmlJSTools et al)
and we later add one of these as a dependency to another plugin (e.g.
make QmakeProjectManager depend on QmlJSTools), which resulted in the
previously enabled plugin being implicitly disabled.

Enabling a plugin in About Plugins now asks for all required
dependencies to be enabled as well.
Disabling a plugin in About Plugins now asks for disabling all plugins
that require it.
Using the -noload command line option now disables all plugins that
require it in addition.
Using the -load command line option now implicitly enables all plugins
that are required.
Multiple -noload and -load options are handled in the order given on the
command line.

Task-number: QTCREATORBUG-9131
Change-Id: I0956106105060a7898a8992e0629009d5ec3ea4d
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>