mapillary plugin: show download progress

[richlv]

When Mapillary images are set to manual download, it is often unclear whether there are no images in an area, or whether the download is slow.
It would be great to have some non-modal status indicator, showing when download starts / is in progress / has finished.

[richlv]

While something similar to a status message seems to be displayed in the lower right corner, it is unclear whether multiple downloads can happen in parallel, and sometimes initiating the download does not show any message there.
The same field also shows messages about selection and other things, thus it is hard to figure out the Mapillary image download status there.

[skyper]

Even without manually download and a slow, unstable connection, I often do not know, if/when images are downloaded. Some indication about download progress would be nice. Right now, I always need to look at the console output in order to get some information.

[taylor.smock]

This is fixed in Mapillary master, with a Download Dialog (thanks to the GSOC student, GhostFoxSledgeHammer (Kishan)).