Add "sticky" map download reminder

When an online map download is aborted (because of tile limits or network problems or just manually), it is quite hard to restart after some time has passed. You have to

a) remember that there was a pending download

b) remember what map you tried to download

c) find that map again in the map list and hit the download menu

Not very intuititive and easy to forget. I suggest that Locus shows a notification in the notification area automatically whenever a map download is pending... even if quit/restarted/rebooted multiple times and even if it was a long time ago. That notification would look like

Locus pending map download:

<map name>, started <startdate>, aborted <abortdate>, <x>/<total> tiles downloaded, <y> tiles missing.

When clicked, obviously the map download is resumed.