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/source_dialog.cpp

@@ -642,6 +642,13 @@ gboolean pick_probe(gpointer user) {
     gtk_editable_set_text(GTK_EDITABLE(c.form_note), "made by the new-host probe");
     adw_spin_row_set_value(c.form_buffer, 32);
     adw_switch_row_set_active(c.form_relative, TRUE);
+    // 8/8: the form's row titles are mnemonics (`_Server`, `_Port`, `Multicast _group`, ...). The two
+    // buttons already asked for it; the rows did not, and drew their underscores.
+    std::fprintf(stderr, "[sources] form mnemonics=%d/8\n",
+                 (row_underlines(G_OBJECT(c.form_server)) + row_underlines(G_OBJECT(c.form_port)) +
+                  row_underlines(G_OBJECT(c.form_group)) + row_underlines(G_OBJECT(c.form_iface)) +
+                  row_underlines(G_OBJECT(c.form_name)) + row_underlines(G_OBJECT(c.form_note)) +
+                  row_underlines(G_OBJECT(c.form_buffer)) + row_underlines(G_OBJECT(c.form_relative))));
     std::fprintf(stderr,
                  "[sources] form: name='%s' note='%s' buffer=%.6g relative=%d "
                  "connect_sensitive=%d\n",