Shared scroll position for different text views

[Karagy]

For example, we can see shared scroll position between "Display character" view and spell description frame in spell manager.

How to reproduce:

1. create character with any spell with long spell's description.
2. call "Display character" via 'd'. Scroll via Up Arrow or PgUp key to see text top.
3. leave "Display character".
4. call "Spell manager" via 'm'.
5. choose spell with long scrollable description.
6. scroll spell description via PgDn key for few lines.
7. leave Spell manager and return to "Display character".
8. At this moment, displayed text also scrolled up for same number of lines as in last scrolled spell description.

It's not a bug. And, even, not req for enchancement. Just care about (possible) some hidden bug.

[RMTEW FULL NAME]

The scroll position is now cleared when the scroll area is cleared, which means that where things are scrolled to is no longer the same even if something different was displayed in the scroll area. Reported by Karagy. Fixes #125

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