`FreeCam`: disabling the free cam while the mouse is captured prevents user from freeing mouse again.

## Bevy version and features
main @ `abdf657ef`, `free_cam` feature

## What you did
I added a keybind to toggle the `FreeCam`'s `enabled` field.

## What went wrong
When pressing the keybinding while the mouse is captured (either by holding LMB or pressing M), the mouse remains captured and the only way to free it is to re-enable the free cam and then press M or tap LMB again.

