Port PlasmaCore.IconItem to Kirigami.Icon

With frameworks/kirigami!1155 (merged) and frameworks/kirigami!1189 (merged), Kirigami.Icon should now be a drop-in replacement as long as we're okay with only using icons from the icon theme, not the Plasma theme. This comports well with plasma-desktop#82 (closed), which has been agreed to.

So now we need to move Plasma Breeze icons into the Breeze Icon theme. Here's a tool that helps: frameworks/ksvg!14 (merged)

Merge requests porting to it:

• documentation/develop-kde-org!301 (merged)
• frameworks/breeze-icons!268 (merged)
• frameworks/breeze-icons!270 (merged)
• frameworks/breeze-icons!271 (merged)
• https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/838
• frameworks/extra-cmake-modules!369 (merged)
• !3044 (merged)
• plasma-desktop!1635 (merged)
• kdeplasma-addons!438 (merged)
• plasma-pa!197 (merged)
• plasma-nm!275 (merged)
• bluedevil!137 (merged)
• bluedevil@26976727
• kscreen!227 (merged)
• milou!67 (merged)
• kwin!4281 (merged)
• plasma-sdk!90 (merged)
• plasma-mobile!357 (merged)

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To test, apply all MRs and apply the Breeze icon theme and the Breeze Light color scheme. Then check with the following Plasma themes:

• Breeze Plasma theme: no visual changes
• Breeze Dark Plasma theme: no visual changes
• 3rd-party Plasma theme that includes icons: most places where icons from the Plasma theme were previously seen now instead display Breeze icons (not all, since there are a few cases where icons were displayed using raw SVG items which need to be ported as well, but that work is outside the scope of this specific Issue)