Introduce a command line option to disable any kind of global shortcuts

Summary:
This command line option is useful for KWin in embedded use case. That
is when KWin is just used as a compositor for one application instead of
a complete desktop environment. In such a setup global shortcuts are not
wanted and interfere with the application. E.g. one does not want Alt+F4
to close the window, that would render the system unusable.

This change introduces a command line option and disables the following
event filters and spies:
 * global shortcuts
 * modifier only shortcuts
 * terminate session
 * virtual terminal switching
 * screen edges

KGlobalAccel still gets inited, otherwise the (non-functional) binary
would be launched when KWin registers it's global shortcuts.

Test Plan:
New test added based on existing tests for the global
shortcuts, ctest passes

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17304