[PATCH 2/6] common: Always let QVariant::fromValue() deduce the type

[PATCH 2/6] common: Always let QVariant::fromValue() deduce the type

From 41bf70c263ee0af80ad1850fabe77ffffee188f4 Mon Sep 17 00:00:00 2001
Bug: https://bugs.quassel-irc.org/issues/1544
Bug-Ubuntu: https://bugs.launchpad.net/quassel/+bug/1885436
Origin: upstream, https://github.com/quassel/quassel/pull/518
Origin: upstream, https://github.com/quassel/quassel/commit/41bf70c263ee0af80ad1850fabe77ffffee188f4

In some places, we would explicitly give the type to
QVariant::fromValue(), forgoing automatic type deduction. This is
almost always redundant, and in the cases where it is not, the
input type should be explicitly cast.

Additionally, the implementation of QVariant::fromValue() subtly changed
in Qt 5.14, leading to compile errors when giving a non-decayed type
(as we did in SignalProxy::attachSignal()).

Gbp-Pq: Topic qt514
Gbp-Pq: Name 0002-common-Always-let-QVariant-fromValue-deduce-the-type.patch