dictextension dict

Add code to notify the user when their preferred database is not on the server.
Also fix a problem where "Search All Databases" would become unselected when the databases were found.

Add storePreferenceToField, a helper for writing a preference to a hidden field in the DOM.

Simplify the callback parameter for getting the list of databases.
There's no need to create a class of objects to receive the callbacks.
A simple object is sufficient.

Fix a UI annoyance where, if the user selected a server-specific
database, the database would be set to 'Search All Dictionaries'
until the response from 'SHOW DB' returned.

In order to show what the user previously selected, we now store
both the database name (the part that DICT cares about) and the
human-readable database description.

Fix two mistakes made in the previous revision
1) Only select a database if the host/port match the default
2) Remove the now-defunct updateSupportedDatabases

Inline updateSupportedDatabases into its only caller.
This simplies to overall code and removes some code duplication.

Fix a bug where an error message (or list of databases) could
be shown for a database that was previously selected, but is
not currently selected. This happens most often when selecting
a slow server, followed quickly by selecting a fast server.

Use appendItem to populate the server listbox.

Add a menuseparator node between the default database choices,
which are available on all DICT servers, and the server-specific
choices.

Attempt to change the file type of a XUL file so that a diff is possible.