gtk-ng: prevent split button from becoming focused (#8136)

Without this, if you create a new tab by clicking on the split button it
will be the focus for any new input. So for example if you create a new
tab and then immediately press the space bar a bunch of new tabs will be
created. Other keypresses will just "disappear". Only by clicking in the
new tab to focus it will keyboard input go to "the right place".

src/apprt/gtk-ng/ui/1.5/window.blp

@@ -50,6 +50,8 @@ template $GhosttyWindow: Adw.ApplicationWindow {
           tooltip-text: _("New Tab");
           dropdown-tooltip: _("New Split");
           menu-model: split_menu;
+          can-focus: false;
+          focus-on-click: false;
         }
 
         [end]