Touch up signature faces

[wyuenho]

1. We should not use font-lock-comment-face as the signature posframe border color. There are actually proper faces that frame borders use for this purpose, so let's just use them so we can have a child frame that has borders that look like every other child frame. These are also faces posframe-show will set the border color on.
2. When propertizing the parameter hints highlight in the signature string, instead of using the eldoc-highlight-function-argument face, let's use a new LSP-specific face that by default inherits from eldoc-highlight-function-argument so users can theme it.

[jcs090218]

Make sense! Thank you!