Use the correct cursor when hovering over the editable part of a spinbox

Summary:
Use the I-Beam cursor when hovering the mouse over the editable part of a spinbox,
just like QWidgets spinboxes do.

Test Plan:

Reviewers: VDG, Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18647