[GTK3] Add styles for libhandy widgets

Summary:
There are now styles specifically for libhandy widgets to make them
better resemble Plasma's mobile widgets.

Test Plan:
Before:
Dialpad:

Switcher:

After:
Dialpad:

Switcher:

Reviewers: Breeze, VDG, ndavis

Reviewed By: Breeze, VDG, ndavis

Subscribers: ngraham, ndavis, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24786