Add the description tag to the generated pkgconfig files

Summary:
pkgconfig complains when the .pc file doesn't have a description. eg:
$ pkg-config Baloo
Package 'Baloo' has no Description: field

With this change, if the DESCRIPTION parameter is not used,
ECM_GENERATE_PKGCONFIG_FILE will :

• First look if there's a metainfo.yaml file and get the description from there
• If the file doesn't exist, or if the description tag is empty or non-existent,

create a description based on the LIB_NAME value

Test Plan: Tests added & pass

Reviewers: dfaure, apol

Reviewed By: dfaure

Subscribers: Frameworks, Build System

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D9056