Add contact dialog: add tool tips

To help users understand the purpose of the various fields of this
dialog, tool tips are added by this commit. They provide basic
explanation for the primary controls (name, address/number, account and
group).

Thanks to David Bolton for suggesting the used tool tips text.
Add contact dialog: support prompts in text fields

This commit adds a function which makes it possible to add faint gray
prompt to a text field. The foreground color of the prompt's text is the
one of the text field, but half transparent. As soon as text is added to
the text field, the prompt will vanish and it will re-appear when the
text field is emptied again.
Add contact dialog: have prompts in name and contact fields

Having prompts in those two fields gives the user a hint of what (s)he
is expected to put into them. This commit will use the example name
'Lisa Simpson' and the example contact 'lisasimpson@jit.si' for both
fields. The text has a faint gray color and will vanish if the user adds
text into the field.
Add contact dialog: focus name field by default

Usually, it is the name what a user has in mind first when adding a new
contact. Therfore and because it is the first field in the dialog, it
makes sense to focus this text field by default.
Add contact dialog: update account and group labels

The former "Select account" and "Select group" labels have been changed to just
"Account" and "Group". This fits more to to other options which say e.g. "Name"
instead of "Type name". Thanks to David Bolton or this suggestion to make the
naming more consistent.