Inject mouse clicks from SNI to xembedded icons with XTest

Summary:
A certain toolkit doesn't register for mouse press release events
because it now uses XI2 only.

Injecting those directly to the window is too difficult, fortunately in
the GTK3 case we can use XTest to send an event. Something I had
previously chosen against using because it didn't work with something
else (can't remember what). I now have a bit of code choosing which
method to use, which will hopefully cover all cases.

Code is a bit convuluted because the xcb version of xtest doesn't have
the high-level method I want to use in it's API, so I just used Xlib
version.

CCBUG: 375017
CCBUG: 362941

Test Plan:
Ran my usual bunch of test apps:
- xchat
- a GTK3 systray demo I made
- skype (A Qt4 app without SNI patches)

All worked as before

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5156