[info] use static form of CGUIInfoLabel::GetLabel where we have no object
[info] move logic for determining a portion's contribution to CInfoPortion
[info] use iterators over the info portions
[info] split CGUIInfoLabel::CInfoPortion::GetLabel() into NeedsUpdate/Get
[info] (re)build the infolabel from portions only if it's changed.
[cosmetics] indenting
[stdstring] switch fallback to std::string
[stdstring] replace CStdString with std::string in GUIInfoTypes
Merge pull request #4919 from jmarshallnz/guilabel_update

More efficient info label assembling