Fix issue with `min_samp` becoming of `float` type

It turns out `round(x, 0)` might return value of `float` type leading to
`TypeError: min_samples must be an instance of int, not float.`. Let's
take care of that and also guard user input for minimum of 3 samples.

Indicate receiver heading with gray dashed line

as it is handy for the prospective operator to see where antenna array
is pointing.

Enable Scene Mode Picker

and enable scene mode selection widget since 2D mode becomes mostly
usable now.