icons: Warn on missing icons

icons: Warn on missing icons

Provide new helper functions icon::get() that replace the uses of
QIcon::fromTheme. These functions still use fromTheme() internally,
but log a warning if an icon could not be found. This should make
it easier to detect problems with icons.

Replace all uses of QIcon::fromTheme() with icon::get(), remove
useless fallbacks as that should be taken care of by the normal
icon loader mechanism.

Update the icon import script accordingly.