[processing] native save selected features algorithm

[nirvn]

Description

The collateral "damage" of an experiment: native save selected features algorithm.

@nyalldawson , all good?

Checklist

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[nyalldawson]

Code looks good!

I'm wondering about this behaviour though "If the selected layer has no selected features, all features will be added to the resulting feature". I realise the old alg does that too, but is it really what we want here? Doesn't this behaviour potentially make model behaviour random? (e.g. if a model does a 'select by expression' and nothing matches, then it saves the result and gets everything output)

[DelazJ]

• If the selected input(?) layer (just to avoid too much selection in the sentence)
• added to the resulting feature layer

[nyalldawson]

Good move