De-duplicate code and reduce the amount of code that has to be changed
when new BorderShapes are being added. This also adds and uses the
missing HasBottom().
When the chosen preview border shape has no left and/or right border,
the minimum total preview window size decreases. But due to the
hardcoded value for the minimum size of the preview window the size
could not be decreased further than 5.
The minimum window height decreases when no extra line for the
horizontal separator is used (e.g. with `--info=inline --no-separator`).
In this case the preview window should be able to occupy this extra
line.
Enable resizing the preview window by dragging its border with the
mouse. This works with all border styles except for `none`.
Counter-intuitively, having the border only on the opposite side of the
window works too - dragging from it will first decrease the preview size
to its minimum.
Julian Prein