#73 Add other file checking capabilities

[fdar]

It is currently possible to look for duplicates in bib
files.
I think it would be nice if we could also look for:
- missing pdf and ps files
- non-valid url (wrong syntax, not responding, etc.)

[Morten Omholt Alver]

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Both of these should be simple enough to implement. Do you have any
thoughts on how this should be presented to the user? Iterating through all
entries with broken links (presenting options for correcting the problem?), or
selecting all of them at once? Another option is perhaps to show a "broken"
PDF icon for these entries. It is also possible to "schedule" this check so it is
done automatically once in a while.

[fdar]

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Iterating through all entries with broken links
(presenting options for correcting the problem?),
or selecting all of them at once?

If the check is launched manually, I would go for the first
option to keep the behavior of JabFred coherent: "Find
duplicates" iterates, so my opinion is that "Find missing
pdf and ps files" and "Find broken url" should behave the
same way.

Another option is perhaps to show a "broken"
PDF icon for these entries.

I think this is compatible: After a check has been
performed, I guess users will appreciate if JabRef
emphasizes the broken files/urls with icons.

It is also possible to "schedule" this check
so it is done automatically once in a while.

May be useful too. One can imagine a check after entering a
new reference. BTW, I am wondering if the current version of
JabRef checks for duplicates when a new reference is entered...