Fix marking a video with a resume point as watched.

Although the video was being marked as watched, the resume point wasn't cleared.
CFileItem::GetPath() returns the virtual library path, causing
CVideoDatabase::GetFileId() to return -1 as it couldn't find any actual file and
CVideoDatabase::ClearBookMarksOfFile(int) was never called.
See: http://forum.kodi.tv/showthread.php?tid=207213

This commit also fixes an incorrect use of SetPlayCount(1) when marking as
watched by replacing it with IncrementPlayCount().
Merge pull request #5616 from anaconda/fix-markwatched-path

Fix marking a video with a resume point as watched.