#5 Add validation to all forms

[Mike King]

We need to add validation to all forms in Blogcaster.
Currently, we just assume that the user has entered the
correct data. Here's a short description of the validation
required on each form:

Main Screen:
- Make sure the user has entered text into the Title and
Content fields.

Settings Dialog:
- URL is required - must be a valid http URL
- Username is required - must be > 1 chars
- Password is required - must be > 1 chars
- Blog ID must be selected - ID >= 1