feat: the cursors panel is a UI file

src/ui/cursors-panel.ui holds the panel's shape -- the header with the mode dropdown and the reset
button, and the grid the two cursor times are read in. What is not in it is the grid's contents,
which follow the cursor mode and the rows and are rebuilt whenever either changes.

Panels need a builder that outlives them, unlike the dialogs, which are rebuilt on every opening: a
widget belongs to the builder that built it, so App::ui_builders holds each panel's for the life of
the window. ui_load() is that, in ui_widgets.cpp.

make test-ui also checks that udpscope-ui.gresource.xml lists every .ui file now. That was the third
trap of this refactor -- a file on disk but not in the resource is a Gtk-ERROR at load, which aborts
-- and it is a compile-time mistake that nothing else in the build could see. Verified by unlisting
the new file and watching the check fail, then restoring it.

[cursors-probe] reads as it did: all four modes, pane=1, the timings and the drag cases.

tools/check_ui.py

@@ -43,6 +43,7 @@ answering "fine" without having looked is the one answer it must not give.
 
 import argparse
 import pathlib
+import re
 import shutil
 import subprocess
 import sys
@@ -113,6 +114,17 @@ def main() -> int:
                 for line in messages:
                     print(f"        {line}")
 
+    # Every file has to be in the GResource. One that is not is a `Gtk-ERROR` at load -- an abort rather
+    # than a window that fails to open -- and nothing else in the build notices, because the xml is what
+    # `glib-compile-resources` reads and the file on disk is not it.
+    gresource = directory / "udpscope-ui.gresource.xml"
+    if gresource.exists():
+        listed = set(re.findall(r"<file[^>]*>([^<]+)</file>", gresource.read_text()))
+        for ui in files:
+            if ui.name not in listed:
+                print(f"  FAIL  {ui.name} is not listed in udpscope-ui.gresource.xml")
+                failed += 1
+
     print(f"check_ui: {len(files) - failed} ok, {failed} failed")
     if advice_count:
         print(f"check_ui: {advice_count} pieces of advice — not failures; --lint shows them")