Deprecate QIcon dependency in plasma_window_management protocol

[romangg]

• It makes testing difficult since QTEST_MAIN is needed instead of only QTEST_GUILESS_MAIN. That fires up QtWayland and by that makes autotests less predictable.
• It makes usage of Plasma with non-Qt-compositors impossible, insofar it is also a blocker for Remove Qt dependency #1.
• It is easy to transfer icons pixel by pixel. See for example XFixesGetCursorImage.

[romangg]

One option would be to replace the whole protocol with wlr_foreign_toplevel_management_unstable_v1 and some smaller custom extensions for VDs and so on.

[romangg]

Another option is to do it like wlr_foreign_toplevel_management_unstable_v1 and only send the app id such that the client can guess the icon from that.

From what I know there were issues in the past with this approach and XWayland clients that lead to the QIcon dependency in our protocol in the first place. So this needs some more investigation.

But wlroots does it this way only and it's apparently enough. Also all XWayland clients I tested sent an app ID, which were:

• firefox
• chromium
• gimp
• xeyes
• neverball
• steam
• xfig