nemo-terminal: F4 toggle doesn't work when VTE terminal has keyboard focus

Steps to reproduce:

1. Open Nemo
2. Press F4 to open the embedded terminal
3. Type something in the terminal (terminal has keyboard focus)
4. Press F4 again to close the terminal

Expected: Terminal closes (toggle).
Actual: F4 is consumed by the VTE terminal widget; nothing happens. Must click outside the terminal first to move focus to the file list, then F4 works.

Comparison: Dolphin's embedded terminal (Konsole) handles this correctly — F4 toggles regardless of where the focus is.

Root cause: In nemo_terminal.py:_toggle_visible() (line 572), the key-press-event is connected to the Nemo window with connect_after, but when the VTE terminal widget has focus, it consumes the key event and it never reaches the window handler.

Suggested fix: Either:

• Add F4 as an accelerator on the window (not just a key-press handler), so it works regardless of focus
• Or use a global keybinding that's not consumed by VTE
• Or connect the key handler on the terminal widget itself instead of the window

Tested on Linux Mint 22.3, nemo-terminal 6.6.0.

Start in nemo_terminal.py at _toggle_visible() and inspect how the Nemo window receives F4 while the VTE terminal has focus. Reproduce the listed steps, then evaluate the existing window key handler and the suggested accelerator or terminal-level handling. Done means F4 opens and closes the embedded terminal without requiring focus to move elsewhere.