Reduce the amount of objects that are compiled several times

Summary:
We build some objects several times which makes it uncomfortable to develop KWin
since every time we modify something a lot of things get rebuilt. This should
help a bit although it doesn't solve all the problems.

Test Plan: Builds, tests pass

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: davidedmundson, zzag, anthonyfieroni, iasensio, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D28445