fix: open the window picker row properly

There were some variables that I missed in f620bd2 (fix: don't crash
when opening settings), which caused the window picker rows in "Custom"
and "Blacklist" pages to not open properly. This adds the missing
`declared` keywords to those variables to fix the issue.

Fixes: #5

src/preferences/pages/blacklist.ts

@@ -28,7 +28,7 @@ export const BlackList = GObject.registerClass(
         private declare _add_row_btn: Gtk.Button;
 
         /** Store value of settings */
-        private black_list!: string[];
+        private declare black_list: string[];
 
         _init() {
             super._init();

src/preferences/pages/custom.ts

@@ -29,7 +29,7 @@ export const Custom = GObject.registerClass(
         private declare _custom_group: Gtk.ListBox;
         private declare _add_row_btn: Gtk.Button;
 
-        private _settings_cfg!: CustomRoundedCornersCfg;
+        private declare _settings_cfg: CustomRoundedCornersCfg;
 
         _init() {
             super._init();

src/preferences/widgets/app_row.ts

@@ -40,7 +40,7 @@ export const AppRow = GObject.registerClass(
         private declare _revealer: Gtk.Revealer;
         declare _expanded_list_box: Gtk.ListBox;
 
-        private bind_property_handler?: GObject.Binding;
+        private declare bind_property_handler?: GObject.Binding;
 
         /** Store event handlers for this widget */
         private cb?: AppRowHandler;