Find file dialog improvements: support ignore mask instead of directories, improve help page

[igorp1024]

1. It is better to make Find file dialog to support ignore file mask instead of directories. It will prevent search with content not in just directories (.git, target) but in files also (*.jar, *.class)

2. It is not obvious from find file dialog context help page how to separate multiple directory names. I think it worth mentioning in help page that semi-colon is a name separator char.

Thanks!