Add SHA-1 and SHA-256 digests

[papjul]

Checklist

• I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
• I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• I have taken the time to fill in all the required details. I understand that the request will be dismissed otherwise.
• This issue contains only one feature request.
• I have read and understood the contribution guidelines.

Feature description

Properties dialog already shows MD5 value. Adding SHA-1 and SHA-256 digests, as two other popular digest options, so we don’t need to rely on another app to just verify the checksums of what we download. Other digests may be available in the Java implementation, but only MD5, SHA-1 and SHA-256 are guaranteed to be available on the Java platform, so I didn’t care to implement others (I know that KDE file manager implements SHA-512 as a 4th option, for example, but I never saw it used).

Why do you want this feature?

To verify my downloads when a MD5 digest is not provided (and only SHA-1 or SHA-256)

Additional information

Implemented in PR FossifyOrg/commons#88