Terminal > Process input + Focus implemented

[tmontes]

Major progress towards completing #5:

• Process windows are focusable.
• CTRL-F changes focus: cycles between process windows and None.
• When a process window is focused, terminal input is written to the process PTY.

Pending:

• Need to review low-level I/O loop terminal input handling: currently preventing from sending q and D to process PTYs.
• Terminal cursor visibility: mostly not working.
  • Should not be visible when there are no focused process windows.
  • Should be visible on the focused process window, if visible in that context.
• Somehow visually indicate focus changes such that users know which window/process is focused.