Added missing QStringLiteral to Dolphin code

Summary:
Changed
```
QIcon::fromTheme("icon")
```

to

```
QIcon::fromTheme(QStringLiteral("edit-entry"))
```

as [[ https://phabricator.kde.org/D16019#339284 | discussed here ]].

Reviewers: ngraham, #dolphin, elvisangelaccio

Reviewed By: ngraham, #dolphin, elvisangelaccio

Subscribers: elvisangelaccio, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D16050