Add "Open with" right click item and dialog (fixes #12)

@bluesabre, I had asked you a while back whether I should use the Gtk App Chooser (works with multiple files, but doesn't have custom command entry) or the Thunar App Chooser (works only with single files, but has custom command entry).

Unfortunately I got disconnected and must've missed your message. I just went with GtkAppChooser dialog for now as it works in all cases -- whether people have Thunar installed or not, and when trying to open multiple files. I tried using the Thunar one when possible, but using two file choosers or having it only work on single files felt a little half baked.

Anyway, if you'd like it a different way or see any problems let me know!

Thanks!

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Adds an "Open with..." menu when right clicking on results. This brings up a GtkAppChooserDialog where you can select which program to open the file with.

Also added the default application name to the "Open" right click menu entry.

Will work with multiple selected files. Zip file contents will extract to tmp directory and the tmp files will be opened (same as normal Right Click -> "Open"/"Save As...").

This required importing Gio from gi/gi.repository.

Also fixes a zipfile error, where Right Click -> Open on folder in .zip file gave an error. (issue #57)