[BT] Emit the too big file warning only for .torrent files.

Fixes #1778.
[Poshuku] Slightly better code.
[Poshuku] SourceViewer UI fixes.
[Poshuku] SourceViewer dumb find support for #1790.
[Poshuku] Mark source searcher as successful/unsuccessful.

For #1790.
[Poshuku] No need for class-level var for #1790.
[Poshuku] Added open shortcut for #1790.
[Poshuku] Fixed a typo, finally closes #1790.