BASIC exporter should export the active screen, not the first in the list

[Esshahn]

It seem that the BASIC exporter always chooses the first screen to export when there are multiple ones. It would be better if it exports the current visible/selected one instead.

Otherwise one would need to delete all screens that are one the left side of the thumbnails to export the current screen.

Scenario below exports the first screen, not the second one with the white border

[nurpax]

It was intended to export the currently selected screen, so definitely a bug.

[nurpax]

OK, here's the bug:

const saveBASIC = (filename, fbs, options) => {
  try {
    let lines = []
    // Single screen export
    convertToBASIC(lines, fbs[options.selectedFramebufIndex], 0)

    let backgroundColor
    let borderColor
    if (fbs.length >= 1) {
      backgroundColor = fbs[0].backgroundColor
      borderColor = fbs[0].borderColor
    }

I think I originally exported all the frames and then later decided to export just one. And ehh brainfart^Wrefactored it to pick the wrong border/bg colors.

[nurpax]

Same bug in .asm export too. Fix incoming.