Use lambdas for queued invocation of composition suspending

Summary:
Qt's metaobject is rather sensitive with scope resolution.

Foo::Bar and Bar don't always match to a Qt metaobject, even if they
refer to the same thing to a compiler. Here we register
X11Compositor::SuspendReason but Q_ARG uses SuspendReason and they don't
match. This leads to a runtime failure where the method isn't invoked.

Rather than fixing metaobject usage, port the whole thing to lambdas
which does better compile time checking and is generally nicer to read.

BUG: 412353

Test Plan:
Ran xprop to block compositing. Compositing was blocked.
Grepped source code for Q_ARG use

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D24244