Prevent infinite loop when using on-quit

[joelim-work]

• Fixes lf doesnt close/kill, when closed window in tilling windows manager #1819
• Fixes Bug: on-quit is potentially executed infinite times if in synchronous block #1854

To reproduce, use the following config, start lf and then close the terminal window:

cmd on-quit ${{
    notify-send foo
    sleep 1
}}

Because the terminal is already closed, running on-quit as a synchronous shell command will cause app.quit to be called again, forming an infinite loop.