This change is motivated by the fact that we need to suspend libinput
before switching the virtual terminal. Also we don't want to take over
libinput if we do not have a VirtualTerminal created - in windowed mode
we don't want libinput to be started. So binding it to the backends which
create the VirtualTerminal makes sense.
The KWin::Application gains a new signal virtualTerminalCreated which is
emitted from VirtualTerminal once it's properly setup. This is used by
Input to create Libinput integration instead of binding it to logind.
Furthermore Libinput gets suspended when the VirtualTerminal reports that
it is no longer active. For re-activation we still just use logind's
session active property.
A new Singleton VirtualTerminal is added. It interacts with Logind to
get the VTNr to take over. To get the signal to release and acquire the
VT we use a signalfd with a QSocketNotifier to monitor for signals. The
used signals must be blocked for all threads prior to startup otherwise
they are delivered to secondary threads causing issues.
