Testcases: Use Xephyr instead of XDummy

Replace the XDummy script with Xephyr. This is done because of some
changes in the Xorg server that make XDummy difficult to use.

Rename library internal variables and function names to replace "xdummy"
with "xserver" to show this change (except for renaming the package and
lib file for better git history).

Rename the switch `--keep-xdummy-output` to `--keep-xserver-output`.
This switch should now be rarely used because Xephyr requires less set
up.

Replace "xdummy" with "xephyr" in comments and utility help
information. Update docs to show the new dependency.

fixes #1367
t/114-client-leader: fix race by waiting until the window is mapped