Split the sidebar into Chain and Setup tabs, with a draggable chain list

The sidebar grew crowded as the chain got longer. The chain and the params
of the selected stage now share a Chain tab through a vertical split the
user drags, and the sources, canvas and seed move to a Setup tab. Stage
rows are compact, with their move and remove buttons shown only on the
hovered, selected or focused row.

Add effect had stopped opening: its menu of fifteen effects no longer fit
below the button, and GTK does not show a popover that does not fit. The
menu now scrolls past 480 pixels, and the button sits beside the chain
heading, where the split cannot push it down the screen.

The window code becomes an effector_gui library so a new GUI test binary
can drive the real window under Xvfb; ctest skips it when xvfb-run is
missing.

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

tests/CMakeLists.txt

@@ -5,3 +5,19 @@ target_link_libraries(effector_tests PRIVATE effector_core)
 target_compile_options(effector_tests PRIVATE -Wall -Wextra -Wpedantic)
 
 add_test(NAME core COMMAND effector_tests)
+
+# The interface drives the real window, so it needs a display. Under ctest it
+# gets a private Xvfb one when xvfb-run is installed; without it, or without a
+# display of its own, the test reports itself skipped rather than failed.
+add_executable(effector_gui_tests gui_tests.cpp)
+target_link_libraries(effector_gui_tests PRIVATE effector_gui)
+target_compile_options(effector_gui_tests PRIVATE -Wall -Wextra -Wpedantic)
+
+find_program(XVFB_RUN xvfb-run)
+if(XVFB_RUN)
+    add_test(NAME gui COMMAND ${XVFB_RUN} -a -s "-screen 0 1400x900x24"
+             env -u WAYLAND_DISPLAY GDK_BACKEND=x11 $<TARGET_FILE:effector_gui_tests>)
+else()
+    add_test(NAME gui COMMAND effector_gui_tests)
+endif()
+set_tests_properties(gui PROPERTIES SKIP_RETURN_CODE 77)