feature: add point manually

[ilf]

Sometimes (when travelling in metal objects like planes, busses, trains) TTT doesn't get a good fix for a long time. It would be awesome to be able to manually add points (like bus stops) afterwards. This is different from #12 which would like to set points with having a fix, I would love to set points without needing a fix. Currently, I have to export, edit and import the GPX.

Thanks!