New configuration property for initial zoom scale on GUI

[GabrieleC]

Description

This PR add a new configuration property in jmeter.properties file to set the initial zoom scale for the GUI.

Motivation and Context

This option is useful to avoid resetting zoom scale manually every time GUI is started.

How Has This Been Tested?

Tested on Windows 10:

• open JMeter and notice the initial zoom level
• close JMeter and set a value greater than 1 on the new property gui.initialZoomScale
• open JMeter and notice the greater zoom level applied at startup

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• [X ] My code follows the [code style][style-guide] of this project.
• I have updated the documentation accordingly.