b3270 thumb Indication

### thumb Indication

Scrollbar thumb has changed.

Attribute | Always present? | Purpose
--- | --- | ---
top | yes | Fraction of scrollbar to top of thumb (0.0..1.0)
shown | yes | Fraction of scrollbar to for thumb display (0.0..1.0)
saved | yes | Number of lines saved
screen | yes | Size of a screen
back | yes | Number of lines scrolled back

Example:

    <thumb top="0.01031" shown="0.98969" saved="43" screen="43" back="0"/>

b3270: Protocol - Operations - Indications