Allow user-defined priorities for the signs that are placed

[mvanderkamp]

There are a lot of plugins using the sign column these days. Even just with neomake, using several linters together can lead to conflicts. It would be ideal if we could customize the priority for the signs, so that, for example, error signs would have greater priority than info signs in case of a collision. It would be even nicer if priority could be set on a per-maker basis. For example in python, this would allow signs for warnings from 'pyflakes' to take priority over those from 'pycodestyle', regardless of the order in which the signs are placed.

[mvanderkamp]

While we're at it, another great enhancement would be for the signs to make use of sign groups in order to guarantee that there would never be a logical (if not visual) conflict with signs from another plugin. It would also make it easier for users to customize their interactions with the signs. If users can define sign groups on a per-maker basis, they could even do things like easily jump between signs for a specific maker, instead of having to do some sort of location/quickfix list filtering scheme.

[blueyed]

So this is mainly implementing/using sign groups and priority with newer Vim/Neovims? (see :h sign-group, and :h sign-priortiy)

For reference: this is how vim-gitgutter used the new functions: airblade/vim-gitgutter@cbaa22e

Related: sign_placelist (vim/vim@809ce4d31) missing in Neovim still.