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

[sporteka2]

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.