Prompt for db username/password if they haven't been set.

Prompt for db username/password if they haven't been set.

This is implemented with a popup widget that is contained within the
OnlineGlomView and managed by the OnlineGlomActivity.

* src/main/java/org/glom/web/client/OnlineGlomService.java: Two new
methods for checking and setting the database username and password.
* src/main/java/org/glom/web/client/OnlineGlomServiceAsync.java: Two
new methods for checking and setting the database username and
password.
* src/main/java/org/glom/web/client/activity/OnlineGlomActivity.java:
Display authentication popup if the JDBC connection to the database
has not been authenticated.
* src/main/java/org/glom/web/client/ui/AuthenticationPopup.java: New
file.
* src/main/java/org/glom/web/client/ui/OnlineGlomView.java: Add methods
for dealing with the authentication popup.
* src/main/java/org/glom/web/client/ui/OnlineGlomViewImpl.java:
Implement the methods for dealing with the authentication popup.
* src/main/java/org/glom/web/server/OnlineGlomServiceImpl.java: Don't
try to executed queries if the database connection hasn't been
authenticated. Implement methods for checking and setting the
database username and password.