Add Duplicate feature

Summary: Adds a Duplicate feature to Dolphin, showing up as a menu item in the File menu that appears when one or more items are selected and the directory is writable. Duplicated items receive the names of the original files with " copy" appended before the file extension, if any.

Test Plan:

{F5201386} {F5201393}

Test cases:

- Try to duplicate when nothing is selected: **PASS**: menu item is grayed out
- Try to duplicate anything on a read-only local volume: **PASS**:  menu item is grayed out
- Try to duplicate anything on a read-only samba share: **PASS**: menu item is grayed out
- Duplicate single local file on R/W volume: **PASS**: item is duplicated and named correctly
- Duplicate multiple local files on R/W volume: **PASS**: 3 items are duplicated, named correctly, and selected
- Duplicate single local directory on  R/W volume: **PASS**: item is duplicated and named correctly, but a rename operation is not initiated
- Duplicate multiple local directories on R/W volume: **PASS**: 3 items are duplicated, named correctly, and selected
- Duplicate single file on R/W samba share: **PASS**: item is duplicated and correctly
- Duplicate multiple files on R/W samba share: **PASS**: 3 items are duplicated, named correctly, and selected
- Duplicate single directory on R/W samba share: **PASS**: item is duplicated and named correctly
- Duplicate multiple directory on R/W samba share: **PASS**: 3 items are duplicated, named correctly, and selected
- Try to undo a successful duplication: **PASS**: operation is undone

This is my first attempt at a big change like this and I'm sure it's full of issues. I will accept any and all suggestions for improvement. :)

Reviewers: #dolphin, #kde_applications, elvisangelaccio, dfaure, broulik, davidedmundson

Subscribers: kfm-devel, meven, markg, fazevedo, cfeck, #dolphin

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D8208