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:

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