Export Bitmap variable replacement logic never triggers

[fuglaro]

https://github.com/TinyCircuits/TinyCircuits-Thumby-Code-Editor/blob/ef59ff908c32a56d6b0b067829abdc39b6edb772/js/bitmap_builder.js#L131

I noticed that the exportBitmap function had some really nice logic that replaces the bytearray for a selected variable. Its pretty nice functionality that never gets triggered because the selected lines are never passed into the function. The importBitmap function gets passed the selectedLines from onImport, but the same isn't true for onExport. The following change probably fixes it, but the onImport function name is confusing there:

         this.EXPORT_LINES_BTN.onclick = () => {this.onExport(this.exportBitmap(this.onImport()))}