LayoutItemField: Fix loading of custom titles.
* src/main/java/org/glom/web/server/libglom/Document.java
loadDataLayoutItemField(): The title, if any, instead of the field
title, is stored in a title_custom node. Load it from there.
* src/main/java/org/glom/web/shared/libglom/CustomTitle.java: Add this
class.
* src/main/java/org/glom/web/shared/libglom/layout/LayoutItemField:
Add getCustomTitle() and use it, instead of super.getTitle*(), in the
getTitle*() overrides.
* src/test/java/org/glom/web/server/libglom/LayoutItemFieldTest.java:
Adapt.