Provide export/save buttons inside toolbar

[de-jcup]

Situation

With #453 we remove the "direct save" action and the possibility to show up as PNG . But there can be usecases where it is necessary to have those files as well

Wanted

User shall be able to save the diagram also as an image file of their choice by a simple click

Solution

The toolbar of ditaa and Plantuml editor will be extended with an export action.

We introduce two export|save buttons:

• PNG,
• SVG

The files will be stored as before inside the "direct save": always be named like the PUML file and the location will be same (maybe configurable in future).

IMPORTANT: The export function MUST generate another adoc_file "...export-PNG.adoc" for PNG - asciidoc shall do the rendering for us. So no conversion or special tools are used, just asciidoctor.