Git: Display dialog to configure user & name

Summary:
Added function to get git config
Added checking of user.email and user.name when committing
Added possibility to configure global git parameters
Added QDialog extension to choose the name and email, with possibility to save the settings as global and validation.
Used the extension to handle the lack of user.name or user.email configured when committing.
Added unit tests for readConfigOption and setConfigOption.
This fixes the Bug 347351

Test Plan:
The new dialog works fine.
Tests pass.

BUG: 347351
FIXED-IN: 5.0

Reviewers: kfunk

Reviewed By: kfunk

Subscribers: mwolff, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D852