fakevim: fix indenting

- the indented region has been off by one line as lineForPosition returns 1-based line numbers but QTextDocument::findBlockByNumber expects base 0
- all lines (including first and last line) have to be indented
- if text is collapsed, then findBlockByNumber has to be used instead of findBlockByLineNumber for getting the block containing a line

Merge-request: 96
Reviewed-by: hjk <qtc-committer@nokia.com>