Respect platform's icon sizes

Use QStyle icon sizes instead of hard coding 16 or 32 pixels.
Using styles that follow KDE settings (every KStyle, e.g.
Oxygen) the icon sizes can be configured by the user in
System Settings.

The size for the icon/details view can be later changed
by the user, overriding the defaults. The icon size for the
places panel is not (yet?) configurable separately.

(Not resolving the bug, see comment in review request)

CCBUG: 301959
REVIEW: 105548