API for visible paragraphs works when contents have blank lines

[lhttjdr]

These test cases are for issue #777.

It shows that firstVisibleParToAllParIndex, lastVisibleParToAllParIndex and visibleParToAllParIndex will throw out "Unreachable code" errors when the contents have blank lines/paragraphs.

However, it not always behaves like that.

• When the paragraphs list start and end with non-blank lines, visibleParToAllParIndex will works correctly even there are some blank lines.
• When the edit area has scroll bar, its behavior has much more complicated pattern. For example, when visible region only contains blank lines, it will throw out errors. After scroll to some non-blank lines, it will work again.

What I know now is that getVisibleParagraphs returns wrong list corresponding to blank lines, so that all those 3 functions above won't work because it's impossible to find a same item in list returned from getParagraphs.

[Jugen]

Thanks, this was covered by the PR fix #795