"Clear Buffer" should flush all relevant panes when broadcast mode is enabled

• iTerm2 version: 3.0.15
• OS version: 10.11.6

Detailed steps to reproduce the problem:

1. Command + T
2. Command + Shift + D
3. Command + Alt + I, then Enter
4. echo foo
5. Command + K

What happened: Only one pane is flushed.

What should have happened: All panes in the current tab should be flushed.

My current thinking on this is there should be a "prefix" keystroke that causes the next command to be broadcast to all panes, if possible. e.g., you do cmd-> and then cmd-k, and the preceding cmd-> causes cmd-K to be broadcast.

changed milestone to %Review for next version