Fix several build system errors

Summary:
This patch fixes two different, but closely related buil system errors.

First one is about making KF5IconThemes and KF5ConfigWidgets really optional as intended. Without this patch, on a system with only Qt5 and other Tier 1 framework libs installed, you will get thw following cmake configure error:

• The following REQUIRED packages have not been found:
• KF5 (required version >= 5.62.0)

(full build logs below)

Second error is revealed after the first one is fixed: plugin/kquickstyleitem.cpp includes ksharedconfig.h, but it cannot be found, because KConfig was not specified as dependency.

qqc2-desktop-style is supposed to be Tier 1 framework, however it depends on several other Tier 1 frameworks (which is forbidden for Tier 1 frameworks?) and optionally depends on some Tier 3 frameworks, so shouldn't it be at least Tier 2 framework? Or even higher?
Quote from https://api.kde.org/frameworks/index.html : "Tier 1 frameworks depend only on Qt (and possibly a small number of other third-party libraries), so can easily be used by any Qt-based project." - this is not true for qqc2-desktop-style.

Test Plan: without this patch, try to build with only Qt5 and Tier 1 frameworks installed and see it fails

Reviewers: mart, Plasma, dfaure

Reviewed By: mart, Plasma

Subscribers: cgiboudeaux, kde-frameworks-devel, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24363