Add option to disable smart scrolling in the subtitles area

[zeromind]

If one clicks on the near the first/last line in the subtitles area,
automagical scrolling occurs, so that the previous/following lines are shown.

That may be the cause for unexpected bahaviour when trying to select multiple lines close to the edge:

1. Select third line visible in the subtitle grid
2. Click-n-hold mouse and move the cursor to the 1st visible line to select visible lines 3 to 1
3. Automagical scrolling kicks in, scrolls up
4. more lines than '3' to '1' selected

expected result

3. -
4. lines 3 to 1 are selected

Usually such scrolling only happens when one moved the cursor out of the area of the list (in every other program i can think of - like filemanagers etc).

There should be an option to disable the scrolling when selecting (with both mouse&keyboard) close to the edge, as that can become quite annoying, in my case because the lines i want to have displayed are being scrolled out of the visible area.

[nielsm]

It would probably make more sense to have two cases:

1. User clicked in grid to change selection. No automatic scrolling happens, unless the user clicked on the last visible line in the grid, and that line is only partially visible.
2. Something else happened that caused the selection/current line to change (such as pushing Enter in the subs edit, accepting a line timing in the audio display) and the newly selected line is close to one of the ends of the visible area, then automatic scrolling should always occur.

The simple rule being: If the user interacted with the grid to change selection, automatic scrolling must not occur.

[Plorkyeran]

Smart scrolling now only happens when the user drags or on mouse up when the row is partially offscreen, so I guess this is implemented (except it's not an option since the old behavior was generally broken).