Make AbstractClient::caption no longer a virtual method
Summary:
The implementation can now be shared, it doesn't differ anymore between
X11 (Client) and Wayland (ShellClient). So instead of code duplication a
shared implementation in AbstractClient.
Test Plan: X11 and ShellClient tests still pass
Subscribers: plasma-devel, kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D7426