D18569: Split out a dedicated InternalClient class

zzag added inline comments.

INLINE COMMENTS

> graesslin wrote in shell_client.cpp:1303
> ok, will update. It means the complete if needs to go. !m_internal is always true for Plasma shell so the second part never mattered.

Hmm, what about internal `Plasma::Dialog`s(e.g. an osd that shows up after switching between virtual desktops)? Won't this cause any issues?

REPOSITORY
  R108 KWin

REVISION DETAIL
  https://phabricator.kde.org/D18569

To: graesslin, #kwin
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