Float as a KWin-managed window instead of a layer surface

The grab bar didn't move the window by mouse, and the corner grip never
resized it (the bar's handler took over its drag). The bar and grip now
start the window manager's move/resize, and Super+LMB/RMB work too.

A KWin rule in kwinrulesrc keeps the window above and unfocused, off the
taskbar, pager and switcher, and remembers its position. KWin now places
the window and keeps it on screen.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/CMakeLists.txt

@@ -85,6 +85,7 @@ qt_add_executable(sloposk
     backend/KWinFakeInput.h backend/KWinFakeInput.cpp
     backend/WaylandKeymap.h backend/WaylandKeymap.cpp
     window/FloatingWindow.h window/FloatingWindow.cpp
+    window/KWinRule.h window/KWinRule.cpp
 )
 
 qt6_generate_wayland_protocol_client_sources(sloposk PRIVATE_CODE FILES
@@ -100,7 +101,7 @@ target_link_libraries(sloposk PRIVATE
     sloposk_coreplugin
     Qt6::DBus
     Qt6::WaylandClient
-    LayerShellQt::Interface
+    KF6::ConfigCore
     KF6::WindowSystem
     PkgConfig::WAYLAND
 )