g.projpicker: Import projpicker from the soon-to-be core module

[HuidaeCho]

Merge after OSGeo/grass#1891

[neteler]

@HuidaeCho pls assign this PR to the grass8 branch

[HuidaeCho]

@HuidaeCho pls assign this PR to the grass8 branch

@neteler I think you meant rebasing to grass8? Or adding a new milestone for grass8?

[neteler]

At time is is submitted to addons base branch grass7 (see above). I assume it should be changed to grass8, e.g. by using the "Edit" button in the Web interface next to the title.

[wenzeslaus]

I think there is also some confusion about what base is being changed. The branch "against" which the PR is made is called base in GitHub interface (see the Edit button for a PR). However, we have also operation rebase (in Git and potentially also in GitHub PR interface when merging) which re-creates all commits from a branch on top of another commit (typically same base branch, just later commit). These two share a lot in common, so perhaps that's the reason for the same name, but who knows, GitHub does have an unfortunate way of naming things IMHO (namely, a pull request on GitHub is a merge request on GitLab).