Fix AbstractClient::sizeForClientSize

Summary:
The method is supposed to return the AbstractClient's size for a given
client size. That is the size including the window decoration.

The default implementation returned the passed in client size without
adjusting for the decoration. This resulted in ShellClient getting a
wrong size especially when AbstractClient::adjustedClientSize (which
calls sizeForClientSize) was called.

The result of the incorrect size was for example a shrinking of the
window when starting to resize a window.

BUG: 370345
FIXED-IN: 5.8.4

Reviewers: #kwin, #plasma_on_wayland, broulik, subdiff

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3414