adwaita: the theme as a module, dark by default, for GTK, Qt, the portal and the cursor (hq ADR 0208)

GTK 3/4 settings, qt6ct, portals.conf and the default cursor as owned files.
GTK_THEME, GTK2_RC_FILES, the Qt words and XCURSOR_* as environment
contributions. The GSettings keys the portal serves go in the xinitrc slot,
replacing the predecessor's appearance script, and the cursor in the
xresources slot.

Qt is drawn by Fusion with qt6ct's darker palette instead of the
user-repository adwaita-qt, which is no longer developed. qt5ct is dropped. The
fonts are research 026's Inter and JetBrains Mono, and portals.conf routes the
Secret interface to gnome-keyring, which nothing answered.

The tools are appearance (dark or light per audience, switched for the session),
cursor, icons, and portal-check (which backend answers which interface, and why).

modules/adwaita/config/qt6ct.conf

@@ -0,0 +1,29 @@
+[Appearance]
+color_scheme_path=/usr/share/qt6ct/colors/darker.conf
+custom_palette=true
+icon_theme=Adwaita
+standard_dialogs=default
+style=Fusion
+
+[Fonts]
+fixed="JetBrainsMono Nerd Font,11,-1,5,50,0,0,0,0,0"
+general="Inter,11,-1,5,50,0,0,0,0,0"
+
+[Interface]
+activate_item_on_single_click=1
+buttonbox_layout=0
+cursor_flash_time=1000
+dialog_buttons_have_icons=1
+double_click_interval=400
+gui_effects=@Invalid()
+keyboard_scheme=2
+menus_have_icons=true
+show_shortcuts_in_context_menus=true
+stylesheets=@Invalid()
+toolbutton_style=4
+underline_shortcut=1
+wheel_scroll_lines=3
+
+[Troubleshooting]
+force_raster_widgets=1
+ignored_applications=@Invalid()