KStandardAction: store icon names already as QString-compat raw string data

Avoids the need to create new strings on the heap for each generated action.

In theory there might be an issue on unloading libraries and with the shared QString instances somewhere still used then pointing into darkness. But for one unloading libraries might not really be used on the target platforms, and also is not seriously taken into account in general, so IMHO also no need to do here then.

Edit: actually some code path for special setup already use QStringLiterals for icon names, so change in policy here.