Include coin.png as a plugin resource

[lexiccn]

Does not have an associated issue

• Includes coin.png from FlatIcon as a resource file in the plugin
• Changes town_costs_icon.url to town_costs_icon.path which now supports URLs and filepaths
• Forces reset of this config option due to the change
  • This is intentional due to a debatable rights issue with the original icon
• Falls back to default resource png if URL is invalid or filepath is invalid
• Uses default resource png without message if config is empty (this is the default)

The original icon has rules against hotlinking, which this plugin technically does not do as the icon is only downloaded on reload of the plugin - However it's clearly against the spirit of the rules and does download excessively, so I've changed this to an included resource coin.png to avoid this issue. Similarly, this is why I decided resetting the config option was the best solution.

On the upside this allows users to include custom files, not just URLs, and adds a fallback if it is invalid rather than failing to display.