cmake: switch to the FindPoppler module from ECM

Summary:
Bump the ECM requirement to 5.19, which provides a better FindPoppler
than the local copy (which was itself copied from okular).
As result, we can search for Qt5 component specifically, and use the
Poppler::Qt5 target where needed, with no more manual include
directories.

Reviewers: Krita, rempt

Reviewed By: Krita, rempt

Subscribers: rempt

Differential Revision: https://phabricator.kde.org/D5849