Adding focus to the function in the Receiver Expression

[Ivanluv]

fixes #5576
@africanmathsinitiative/developers this is ready for review

[rdstern]

That's great - and amazing too. I checked with @volloholic who thought that the focus would need to be changed individually for each key.
That was because the keys are not user controls. But I get the idea here is that this was just a one-line change, because the receiver IS a user control and the change can be made here.

I suggest perhaps @maxwellfundi could do a final check - but for me this could be merged now. I checked using the calculator and various keyboards and also one of the modelling dialogues. I also checked with and without the arguments selected. It all seems ok. Excellent!