swaybar: don't expand separator_block_width if separator is false
swaybar: don't expand separator_block_width if separator is false

When swaybar receives the following JSON body

    [
        {
            "full_text": "foo",
            "separator": false,
            "separator_block_width": 0
        },
        {
            "full_text": "bar"
        }
    ]

it should not draw any separator or any space between the two blocks.
However, since swaybar calculates that separator_block_width
0 is too small to fit any configured separator, it will override the
separator_block_width with some non-zero value. This patch changes
that such that the necessary separator_block_width is only expanded if
the block has 'separator: true'.

This should be in line to what i3 does, as its documentation of the
i3bar protocol for separator states that "[...] if you disable the
separator line, there will still be a gap after the block, unless you
also use separator_block_width".

1ace/sway,1ace/sway,1ace/sway