[Design] Improve Export function with ratio

[Turtlepurple]

Hey People,

I hope this is a good place to post Improvement-Ideas to the
openstreetmap.org-frontend:

1. When I use the "select area" feature of Picture Export I would be so
glad to be able to keep a certain ratio, like the a4-paper-size-ratio.

This would be the most straight-forward and easy to realize way towards
simply printing a chosen area of OSM on a single sheet of paper.


I've spend some time trying all the available online printing-renders
listed on the "OSM on paper"-wikipage and I think the function on the
main-page is one of the few that work for simply printing a chosen area
on a single sheet of paper.

there's a few other things I would suggest to add/change if there's
interest to do so:

2. I find it non-intuitive to find the export function in a menu called
"share", maybe just call the menu "Share or Export" the icon seems quite
fitting for both

3. Make other layers available for export. "International-Grey" Would be
great for printing on b/w-printers, but it's not supported by
openstreetmap.org at all.
The "Humanitarian" Layer already works well on b/w-printers, but it's
not supported in the export-function.

4. The scale used for export does adjust when I zoom out, but it doesnt
when I zoom in. It would be great to have a tick-box to sync with the
main-zoom or not.
5. Additionally to displaying/adjusting the scale 1:xxxxx It would be
great to just choose the zoomfactor.

6. Additionally to displaying the output-picture size in pixels it would
be nice to see it in mm/inch at the given dpi (I guess that's fix isn't it).
This is really fancy though and might just overload the UI

I hope you like some of my ideas, Thanks for the great work,

Turtle