Add KIconTheme::initTheme() to configure proper icon set as opt-in

by calling

KIconTheme::initTheme()

before the application creation one can hard override the icon engine and the theme.

The function will honor the user settings from the application config with fallback to kdeglobals.

Via some plugin path hacks one ensures our icon engine is used, that implementation detail could be improved in the future with Qt api additions.

But that will not be visible to the user of this function.

The same handling for our application style is available in kconfigwidgets!239 (merged)