fir header dialog information into min width

[ctzsnooze]

fixes #568

@McGiverGim expressed concern that at minimum window width, not all the information in the header dialog was visible.

I rearranged things a bit - margins, rearranging the header, that sort of thing, and it all now fits quite neatly.

The minimum window width is now 930px, a meaningful increase over the previous minimum of 740px. The minimum width could be as small as 900px and still look OK (see below), but at 930px the header dialog sits symmetrically above the main graphical area and looks good. It should still fit into most computer screens at 930px.

Here is an image showing how it looks at 930px width.

This is the previous appearance at about the same width.

This is the new appearance but with a min-width of 900px (this is not the value in the PR, but I included it here in case you think it might be better like that).

We don't really need all the white space at the top but I couldn't figure out how to get rid of it.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
5 Code Smells

No Coverage information
0.0% Duplication

[blckmn]

AUTOMERGE: (FAIL)

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[asizon]

Thankyou @ctzsnooze @haslinghuis!!