[shell] Set FramelessWindowHint on DesktopView by default

Summary:
The default window type is desktop, so we should also tell Qt that it's
a frameless window. Not doing so means that on construction of the
platform surface our plasma-integration plugin will request a server
side decoration for the window. Which means KWin creates a deco and
destroys it again once the flag propagated. But it also means that the
window gets misplaced due to the temporarily added deco.

By setting the flag the plasma-integration never asks for the deco to
be created in the first place and in all my tests so far the window was
positioned correctly.

Reviewers: Plasma, sebas, mart

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1156