Improve CMake handling of GNOME libraries

Summary:
GNOME libraries were being handled in a manner not
as elegant as possible with CMake's built-in pkgconf handling.
This patch uses CMake's built-in pkgconf handling to trim excess
code and improve style (${GTK_LIBRARIES} -> PkgConfig::GTK+3).

Test Plan: See that everything still builds and links as expected.

Reviewers: #plasma, gikari, apol

Reviewed By: gikari, apol

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28086