Pad the formatted datetime

[jeremypw]

Fixes #1538

BEFORE

AFTER (ISO format)

.

[tintou]

That's not good because the headers also have a margin

[jeremypw]

That's not good because the headers also have a margin

Ah, that's a pity - it was the simplest solution 😞 I guess I will have to tweak the render instead.

[jeremypw]

I know that using spaces is not elegant but bearing in mind it will all have to be re-visited on porting to Gtk4 it does not seem worth spending a lot of time on it.

[lenemter]

When using ISO format the date is almost touching the scrollbar which looks weird.

[jeremypw]

Hmm, that's odd - all formats should have one space padding but I can increase it to 2 spaces if you think that looks better. Or maybe there is a fixed-width space char I can use.

[jeremypw]

Now using single unichar emspace for padding. With this there is no overlap even when hovered.