Settings fixed for VS plugin and shell extension.

* Duplicate context menu items for shared folder. Closes #1987
* VS plugin reading settings fixed. Closes #1935
* Option added that restore old shell extension behaviour. Closes #1821
Fixed issue: open commit from file history shows incorrect file changes

The parent guids from filtered revision grid where used, causing incorrect results
Windows 8.1 added to list of supported OS
FormCommitDiff refactoring, all use one constructor