Reduce the hitbox for regular icon hover.

Summary:
This reduces the sub-rectangle within the delegate that reacts
to pointer hover to wrap the icon and label more tightly, which
makes doing rectangle selections - foiled by accidentally
hover-activating an icon - much easier by giving it larger
gutters between icons to start in.

Subtly, it does _not_ change two other hitboxes:
1. Once an icon is in hover state, it can still by dragged by
   grabbing it on any coordinate within the visible frame, as
   it will only become un-hovered once the frame is exited.
2. For drag hover (e.g within spring loading) the hitbox is
   unchanged - the larger hotbox makes it easier to aim with
   e.g. a drag pixmap attached to the cursor.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4860