If virtualtext.vim is autoloaded first, it will link
ALEVirtualTextWarning to ALEWarning. But ALEWarning is not initialized
yet, so it will create ALEWarning, but with no color definition set.
Shortly after, highlight.vim is autoloaded, which would usually link
ALEWarning to SpellCap, but only if ALEWarning is not already set.
However since ALEWarning is already initialized due to the previous
link, we skip this and never actually come around to properly
initializing it.
We fix this by initializing all highlight groups in highlight.vim, thus
satisfying the dependency of ALEVirtualTextWarning being initialized
after ALEWarning.
Fixes#3585
* Use sign-group only on supported vim versions.
The sign-group feature is only available in nvim 0.4.0 and vim 8.1.614.
* Add priority to ALE signs.
This allows users to set a priority to ALE signs to take precedence over
other plugin signs.
A new function is added here which will later be modified for public use
in linter and fixer callbacks. All linting and fixing now goes through
this new function, to prove that it works in all cases.