[Patch] Imagery providers slippy map enhancements

[Don-vip]

Hi,
Please find attached a proposed patch to enhance further the one I submitted in #6825.
This patch allows to:

• display several bounds (useful for imagery providers that have distinct, non-adjacent, bounds)
• display polygon shapes (it's going to be very useful for the French "GéoLittoral" bounds that cover the entire Atlantic coasts, rather than draw a simple rectangle across all the country)

Here's what it looks like:

The Maps wiki page has to be improved as described in the attached XML file, but this change should be backward-compatible with older JOSM version.