Draft: kbuildsycoca: create mimeapps following effective preference

glib apps expect the preferences to be spec compliant in mimeapps.list but we by default use the runtime InitialPreference value. this causes a disconnect in library behavior where kservice might want to open a text file with kate (because of InitialPreference) while glib might want to use kwrite (because of defaults.list -- which we completely ignore).

instead have kbuildsycoca write a kde-mimeapps.list always.

this has a number effects:

• mimeapps.list exists and the deprecated defaults.list will not get loaded
• because mimeapps.list initially is seeded by our InitialPreference (by virtue of having run buildsycoca first) glib apps will implicitly follow InitialPreference now
• undermines InitialPreference after the first run, "fixating" the defaults after initial application