compilation-mode `mouse-face' property is hardcoded to `highlight' face

[Drew Adams]

In compilation-mode buffers (e.g. grep-mode), the `mouse-face' property is
hard-coded to use the `highlight' face. This hard-coding makes it hard for a
user to use a different `mouse-face' in these buffers.

Could we instead use a new face as the `mouse-face' property value in these
buffers, so that users can easily customize the face to be used for
mouseover highlighting there? Currently, one can only customize the
`highlight' face, which affects all buffers.

This change should only require:

1) adding a new face (defface), say `compilation-mouseover'
2) replacing `highlight' by `compilation-mouseover' in `compile.el'

The `highlight' face could still be used to define the initial value of the
new face.

(Why bother to have a `mouse-face' text property that can have different
values, if it is going to be hardcoded here and there?)