i3bar: Do not kill watcher on EOF

Wait for the child process to exit on its own before freeing
watcher-related resources.

i3bar shows the last received status line until the process exits.

Fixes a race condition where the termination signal was sometimes not
received to display a meaningful error message.
Move grab_all_keys to bindings.[ch]

Also add checks for binding input_type to filter bindings that are not
keyboard bindings.
man/i3.man: Update font doc to reflect i3.config