Only prepend path to filename if the user has not supplied a filepath

[zjp]

I was confused at first why supplying a full path to a file was not working. After diving into the source code, I discovered this block. This change ensures that if the user supplies a path to an input file, that path is used. If not, the program will still presume the file is in the current folder.

This also fixes a bug in the GUI version, where picking a file outside of the DP-4 folder would cause the program to try loading /path/to/dp4-ai//path/supplied/by/qtfilepicker

[zjp]

Addresses #8, and probably #5 and #6

[KristapsE]

Many thanks for this, I will look into testing and merging these over the next week!

[zjp]

We are writing a handful of patches; if you do decide to accept them let me know what order you're going to merge them in, and I'll rebase my branches in the same order to keep your git log clean of trash merge commits.

[KristapsE]

Thanks!