[AppMenu Applet] Pass "ctx" as context instead of "this"

We copy "ctx" pointer by value into our lambda but by the time our timer fires, it might be gone already.
Instead, pass "ctx" as context to QTimer::singleShot so it won't fire when it's gone.

BUG: 382386
FIXED-IN: 5.10.5

Differential Revision: https://phabricator.kde.org/D6951