Clean-up, harmonize, and improve StyleBox API

- Make all margin properties follow the same naming convention (their getter and setter too).
- Remove a virtual counterpart of `get_style_margin` from API.
- Allow to override `get_minimum_size` from scripting and remove `get_center_size`.

editor/plugins/bit_map_editor_plugin.cpp

@@ -53,7 +53,7 @@ BitMapEditor::BitMapEditor() {
 	// Reduce extra padding on top and bottom of size label.
 	Ref<StyleBoxEmpty> stylebox;
 	stylebox.instantiate();
-	stylebox->set_default_margin(SIDE_RIGHT, 4 * EDSCALE);
+	stylebox->set_content_margin(SIDE_RIGHT, 4 * EDSCALE);
 	size_label->add_theme_style_override("normal", stylebox);
 }