Highlighting without running multiple binaries

[J7mbo]

Right now we use fyne to draw a rectangle on the screen, overlayed with a transparent window.

Due to fyne-io/fyne#3063, we have to isolate the fyne run-loop in it's own binary, embed the binary in helios, and then run from memory. This is obviously far less than ideal.

Either wait for the above bug to be fixed, which means the highlighter can then be run internally and the run loop just cancelled / restarted at runtime, or else find an alternative method of highlighting a region of the screen.