Add support for colour text and colour backgrounds to the layout list cells.

Add support for colour text and colour backgrounds to the layout list cells.

Only the cell backgrounds are coloured which leaves a gap between the
cells that isn't coloured. I need to figure out a way to set
'style=background-colour:' on the whole column rather than just the
cell.

* TODO: Add a note about colouring the background of the whole column.
* src/main/java/org/glom/web/client/LayoutListView.java: Add a custom
column type (GlomFieldColumn) and a custom cell type (GlomFieldCell) to
render the coloured text and backgrounds. Use GlomField[] for the row type.
* src/main/java/org/glom/web/client/OnlineGlomService.java: Use GlomField[]
for the row type.
* src/main/java/org/glom/web/client/OnlineGlomServiceAsync.java: Use
GlomField[] for the row type.
* src/main/java/org/glom/web/server/OnlineGlomServiceImpl.java: Use
GlomField[] for the row type. Set the text, text colour and background
colour in the GlomField objects as specified in the glom document. Add
method to convert from Gdk::Color to HTML colour string. Cleanup comments.
* src/main/java/org/glom/web/shared/GlomField.java: New file to encapulate
the glom field text, foreground colour and background colour.