Make zoom limits and step adjustable in GraphEdit

[YuriSizov]

Closes #38249.

This also adds an optional label to the toolbar of the GraphEdit to display current zoom level (in percent).

Note: Earlier version of this PR featured a linear zoom option that was removed

2021-06-16_16-38-33.mp4

New settings are neatly grouped. I've also gone ahead and grouped connection lines related settings.

[AaronRecord]

Not sure the linear zoom property is necessary, I'm not sure anyone's ever asked for this and if someone really does need this isn't it possible for them to implement it themselves?

[YuriSizov]

Not sure the linear zoom property is necessary, I'm not sure anyone's ever asked for this and if someone really does need this isn't it possible for them to implement it themselves?

I'm fine with cutting it out of this PR. As for implementing it on the user side, I don't think it would be trivial, no.

[AaronRecord]

Well if all the zoom properties are exposed, couldn't someone make a script that extends GraphEdit and in _unhandeled_input(), they could implement linear zooming themselves and mark the input event as handled? Or would that not work?

[YuriSizov]

PR amended. The linear zoom option was removed as not that desired. Code was rebased against the most recent master, some code style changes were applied as well as some other adjustments to code to better fit the overall state of our codebase (now that I know it better).

Edit: Push below to make sure that the label has a consistent look.

[AaronRecord]

Haven't tested it, but the code looks good to me.

[akien-mga]

Thanks!