[kstyle] Use application attribute instead of reading config file to check whether to draw menu icons

The current code creates a KSharedConfig for every invokation, which is wasteful

There is an application attribute that we can use to check what we want

plasma-integration sets that attribute based on the same setting we are reading here

This way the config is only read when needed

There is one behavior change: plasma-integration only checks kdeglobals, not the app's config file, so overriding the behavior for one app only is no longer possible. However that's not something we expose in any UI anyway