Change the icon of the new connection button

Description

Now, the icon of the new connection button shows the 'new' icon instead 'new file' icon.

Related Issue

Issue !2885

Motivation and Context

it is confusing to use the new file icon.

Screenshots:

Preview on glade

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.