FontEditor: Allow application to launch without a font

Since LibFSAC requires a reified window before loading a font, it
makes sense to have a safe null state for the app.

This lets us stay alive after a failed file request on startup,
handle failure at any point during initialization, and claw back
memory from all our font RefPtrs.

A default startup font or none at all can now be set in FontEditor.ini

Userland/Applications/FontEditor/FontEditorWindow.gml

@@ -23,10 +23,13 @@
 
             @FontEditor::GlyphEditorWidget {
                 name: "glyph_editor_widget"
+                visible: false
             }
 
             @GUI::Widget {
+                name: "width_control_container"
                 preferred_height: "shrink"
+                visible: false
                 layout: @GUI::VerticalBoxLayout {}
 
                 @GUI::SpinBox {