multiple folders are all mixed up

[pirate486743186]

Expected behaviour

When adding multiple folders (command line or drag and drop), the order of the files should follow the order of the folders.

for example:
1 2 3..... 1 2 3..... 1 2 3...

Actual behaviour

All files are mixed together in alphanumeric order.

for example:
1 1 1.... 2 2 2.... 3 3 3....

Steps to reproduce the behaviour

just drag and drop or open from cli, two folders with containing files with same names

MATE general version

I'm still using ubuntu 18.04
I don't think this matters.

[JorgeStolfi]

See #308

[mbkma]

duplicate of #308