tapping on frequency should ask for frequency

[hackerb9]

I've gotten used to the idiosyncratic interface of sig-gen, but after trying to show it to another person, I think it could use some improvement.

Currently tapping on the displayed frequency does nothing. It should pop up the dialog asking to enter the frequency directly.

Also, there is a hollow rectangle icon which confuses people. They think it is a glyph representing a phone screen. I suggest removing the rectangle icon once tapping on the frequency works.