[plugins/qpa] Add support for QT_WAYLAND_FORCE_DPI env variable

Summary:
If a user specifies the QT_WAYLAND_FORCE_DPI env variable, KWin uses
it to force a logicalDPI, just like QtWayland.

Test Plan: Normally sized window decorations

Reviewers: Plasma

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1605