fix: row titles mean their mnemonics instead of drawing the underscore

Fifteen display strings in the two preference forms were written as GTK mnemonics -- `_Directory`,
`_Server`, `Multicast _group`, `_Theme` and the rest -- but the property that makes GTK read them was
never set, so GTK drew the underscore literally: `_Directory` in the row, not a D with a line under it.

Two switches are involved, FALSE by default in each case. `GtkLabel`/`GtkButton` take `use-underline`;
the rows take `AdwPreferencesRow:use-underline`, "whether an embedded underline in the title indicates a
mnemonic". The two buttons in the source form already set it (so their mnemonics worked and showed no
underscore); the thirteen rows did not. All thirteen now do.

The row classes that carry the property are not all related -- AdwEntryRow has its own, while AdwSpinRow,
AdwComboRow and AdwSwitchRow get it through AdwActionRow -- so the probe reads it by name on a GObject
(g_object_class_find_property, then g_object_get) rather than casting to one row type; a cast to a
`GtkWidget*` where the type is only forward-declared does not compile at all.

`_Cancel` and `_Connect` both claimed C in the same form, so Alt+C could only reach one of them: that
marker was unreachable, which is worse than absent because it is in the source to be trusted. `_Connect`
becomes `Conn_ect`, freeing C for Cancel.

[prefs-probe] reads the count (4/4) on the real widgets; check_ui validates the property against the GIR
for every object in every .ui file, inherited properties included.

src/ui_prefs.hpp

@@ -29,6 +29,23 @@
 
 #include "settings.hpp"
 
+/// Whether a row asks for `use-underline` -- the property (default FALSE) that makes a `_` in a row's title
+/// a mnemonic. Without it GTK draws the underscore as a literal character, which is what `_Directory` and the
+/// other row titles did.
+///
+/// Asked for by *name* on a plain `GObject`: the row classes that carry the property are not all related
+/// (`AdwEntryRow`'s `use-underline` is its own, `AdwSpinRow`/`AdwComboRow`/`AdwSwitchRow`'s come through
+/// `AdwActionRow`), so a common getter or a cast to one row type would miss half of them -- and passing a
+/// row as a `GtkWidget*` where the type is only forward-declared does not even compile. A class without the
+/// property answers false rather than crashing.
+inline bool row_underlines(GObject* row) {
+  if (row == nullptr) return false;
+  if (g_object_class_find_property(G_OBJECT_GET_CLASS(row), "use-underline") == nullptr) return false;
+  gboolean on = FALSE;
+  g_object_get(row, "use-underline", &on, nullptr);
+  return on != FALSE;
+}
+
 namespace udpscope {
 namespace ui_impl {