Introduce mutually exclusive productsets: DESKTOP and MOBILE, make ALL == DESKTOP, use SHOULD_BUILD_* variables

Summary: Based on patch from Adam: https://phabricator.kde.org/D5140

Test Plan:

• Build with -DPRODUCTSET=DESKTOP
• Build with -DPRODUCTSET=ALL -> should build DESKTOP too
• Build with -DPRODUCTSET=MOBILE

Reviewers: piggz

Reviewed By: piggz

Tags: KEXI

Differential Revision: https://phabricator.kde.org/D5150