Add <number> to Wayland captions if the caption is the same

Summary:
Bringing another caption feature from X11 to Wayland. If we have
multiple windows with the same caption, starting from the second window
a suffix <number> is added.

E.g. if we have three windows with caption "foo", the naming is:
 * foo
 * foo <2>
 * foo <3>

The change tries to use as much shared code between the X11 and Wayland
implementation. Unfortunately it's not possible to share completely as
the X11 implementation does X11 specific things like editing the visible
name.

By sharing the code the numbering also works cross windowing system.
That is if a window is called "foo" on X11, a new window on Wayland with
caption "foo" will get adjusted to "foo <2>" and vice versa.

The change also eliminates a duplicated signal for captionChanged in
ShellClient (found by test case).

By using the shared implementation on X11 side a bug gets fixed which
got introduced with the support of "unresponsive", this is no longer
considered and the numbering still works even if there is a window which
is unresponsive.

Test Plan: New test case and manual testing

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7425