Add support for horizontal alignment in the LayoutList columns.

Add support for horizontal alignment in the LayoutList columns.

* TODO: Remove item about horizontal alignment. Add item about
improvements to ColumnInfo.
* src/main/java/org/glom/web/client/LayoutListView.java: Set horizontal
alignment on the columns. Use ColumnInfo RPC object get the column
title and horizontal alignment.
* src/main/java/org/glom/web/client/OnlineGlom.java: Update
LayoutListView creation with ColumnInfo RPC object.
* src/main/java/org/glom/web/server/OnlineGlomServiceImpl.java: Create
a ColumnInfo object for every LayoutList columnn. Convert the
FieldFormatting.HorizontalAlignment to the correct
ColumnnInfo.HorizontatlAlignment with the new
getColumnInfoHorizontalAlignment helper method.
* src/main/java/org/glom/web/shared/ColumnInfo.java: New RPC object
to encapsulate column information like alignment and title. This
could be used to set the colour instead of on a per cell field basis.
* src/main/java/org/glom/web/shared/LayoutListTable.java: Replace
column title storage and retrieval with ColumnInfo.