Instead of describing icons via file name or in the themed icons case
via
a string that is a list of mask/color pairs, we have now a class for it.

Icons are now listed in per-plugin *icons.h headers.

RunControl::m_icon was The only place left where an icon property was in
fact a string. This patch changes that member to be a Utils::Icon.

Change-Id: Ibcfa8bb25e6d2e330c567ee7ccc0b97ead603177
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>