Global export PNG settings

[flmn]

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Is your feature request related to a problem? Please describe.
Not a problem, a feature.

Describe the solution you'd like
Have a settings dialog, user can set Zoom, Border Width, Transparent Background, etc. settings globally. Next when user open a file, can use global setting for export.

Describe alternatives you've considered
Save export settings in file.

Additional context
no.

[lvmm]

Merging in #771. Also settings for all export types should be saved, not just png.

Sometimes it is necessary to export multiple pages of a diagram as images e.g. for publication on a website. When export dialogue (file-export as-) is opened it always reverts do default configuration and one has to reconfigure it every time which is time-consuming, annoying and may lead to mistakes.

[stale]

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