[Calculator] Make buttons' text labels scale in size along with the widget

Summary:
The Calculator widget's button labels were not scaling in size as the widget got bigger,
leading to situations where the labels were tiny when the widget was very large.

This patch addresses that situation by scaling the labels size along with the widget.

For best results, also apply D21502.

Test Plan:

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D21503