Step by step I want to restructure the vim files. The approach to
cleanly divide the config by topics works in zsh pretty well, but is too
vague in vim for my liking.
Especially (the very small) functions and autocommands that have the
purpose of setting options or mapping keys or the interplay of multiple
categories together, make it difficult to decide in which file the
snippets should lay.
Apply the following:
1. Switch to `matchadd` as `match` can only be used for **one**
highlight
2. Use an autocommand group for better behaviour when reloading the
vimrc.
3. Use just one pattern as it seems to work always. (Apparently `\%#` is
not matched in normal mode)
As relative line numbers are only really needed in normal mode and in
the current buffer, disable them in insert mode or when the
buffer/window is not focused.
The old `textwidth` value is meant to be subtracted from colorcolumn, to
then add the new one.
Fixes: 41656d28d6 ("vim:autocmd: Reset textwidth value in colorcolumn")
