Warn before opening 5+ translation files

Summary:
The context menu of a folder node in th Summary view of Lokalize
allows openning all the container files.

While this can be useful, accidentally selecting this option on the root
node of a large project (e.g. KDE translations) can be troublesome.

I’ve ripped off Dolphin’s approach for the same issue here: if more than
5 items are about to be opened, ask for user confirmation first.

I’ve used Dolphin’s limit (5), but I’m open to using a higher value for
Lokalize.

Test Plan: I tested the changes manually.

Reviewers: ppeter, huftis, shaforostoff, sdepiets

Reviewed By: ppeter, sdepiets

Subscribers: #localization, huftis, ppeter

Tags: #localization

Differential Revision: https://phabricator.kde.org/D23953