src/output.c: refactor virtual output related functions

src/output.c: refactor virtual output related functions

This commit moves the virtual output related functions
into their own file at `src/output-virtual.c` with its
own include file to reduce `include/labwc.h` bit by bit.

Additionally, it removes the need to keep the
`server->headless.pending_output_name` char array around
by temporarily disconnecting the handler when creating a
new virtual output. This allows to set the output name
right in the `output_virtual_add()` call rather than to
store the pending name until the new output event handler
has been called.

It also makes adding a virtual fallback output easier in
a follow-up PR.