Umap handling improvements

Umap overlay support hat a slightly bumpy start, so I may have put it on the public server too early, but should now be fully usable in single page mode.

Fixes and improvements applied during the last two days include:

• rendering doesn’t fail anymore when certain properties are not found
• per-layer defaults are now taken into account in addition to global ones
• the “circle” and “ball” markers are now rendered properly
• whether a marker icon should be black or white is now reliably detected
• external marker icons referenced by http: or https: URLs are now downloaded and displayed
• dash patterns are supported when rendering lines
• the map title field is pre-filled with the Umap map name
• when the umap file uses a tile layer style supported by this MapOSMatic instance it is auto selected

Things that are still missing or need to be fixed:

• Umap overlay rendering in multi page format
• handling of label display options
• vertical marker positioning may still be minimally too low or high