enchancement: make active terminal easier to see

[eMBee]

it is not easy to see if a terminal is active for keyboard input or not.

compare to playground, which gets a blue border when active. a similar indicator for the terminal would be nice.

(side note: it appears that the active window border in playground does not go away if pharo itself is not selected. i don't know if this is a bug in playground, or if it is simply not possible to know if pharo is selected. if it is the former, then pterm could do better by showing the active indicator only if both the terminal within pharo and pharo as a whole are selected.)