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

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7426