Use read-only checkboxes for boolean field types.

Use read-only checkboxes for boolean field types.

* src/main/java/org/glom/web/client/LayoutListView.java: Create columns
in the CellTable based on the field type. It currently only
distinguishes between boolean and text columns but I'll need to add
support for more types.
* src/main/java/org/glom/web/server/OnlineGlomServiceImpl.java: Set the
column type in the ColumnInfo object. Add method to convert between the
glom field type enum in ColumnInfo and the glom field type in libglom.
* src/main/java/org/glom/web/shared/ColumnInfo.java: Add support for
field type.
* src/main/java/org/glom/web/shared/GlomField.java: Add support for
getting and setting booleans.