Current Search/Incsearach highlight sets the full background color to
yellow (common in the majority of colorscheme), but results in a
distracting highlight.

This PR proposes a less distracting hightlight, using yellow for the
foreground color instead. Moreover style bold and underline help
distinguing the text in case of styled filetype (e.g. markdown or
programming languages).

Added the following highlights classes

* header, which is the same as the standard "Title" highlight target
* Quote
* Code
* Link

incsearch and search have the same hightlight, making it more hard
to distinguish the current selected string among the one matching
the search.

Use underline to hightlight the current selected element

When two or more buffers are open at the same time, the one without
the focus has a blue background which, being more visible, can make
the user think that this is the on-focus buffer, while it is not.

Attempt fix setting Pmenu colors to have the same bg color of the on
focus split buffer, while using a grayed fg color.