Use KIconThemes and KStyleManager if available during build-time

This can benefit platforms like Windows so breeze's icon can be used if the platform doesn't offer icon theme support out-of-the-box.

See also: https://cullmann.io/posts/kde-applications-and-icons/

What it looks like currently under Windows when build with MSYS2:

Note: the goal of this MR has been changed, see the discussion below for details, see the content below for the original MR description

Original description:

This can benefit platforms like Windows so breeze's icon can be used if the platform don't offer icon theme support out-of-the-box.

While Qt 6.7 start offering some sort of icon support by mapping icon names in the platform plugin, the amount of supported icons is still smaller than what breeze-icons offered, thus still a good choice to use breeze-icons.

Within this change, BreezeIcons is still an optional dependency at build-time.

cc @aacid

by the way, is there a way to specific BreezeIcons to use its light or dark variant of icons? Since it doesn't look right if user is using a dark theme (screenshot on Windows 11 with dark theme, Qt 6.7 is used):