OpenFileRequester: Add checking that file exists

[Tristano]

Currently, OpenFileRequester() doesn't carry out any sanity checks. The end user can manually type the name of a non-existing file.

So it falls back on the programmer to check that the actual filename returned exisists.

Since the purpose of this requester is to allow the user to choose a file (ie: an existing file) it would make sense that this command checks that the string of the selections corresponds to an actually existing file.

It should still accept an empty string as a cancel operation, but it might make sense that if the string doesn't match an existing file name the requester doesn't close --- ie: the user has to either chose an existing file or cancel the operation, no compromises.

This would spare programmers from having to implement sanity checks on the user choice.

[kenmo]

+1 for an optional OpenFileRequester() flag

On Windows, I think this is as simple as adding the #OFN_FILEMUSTEXIST flag to an API call.

[marcoagpinto]

+1