Of­fline ac­cess to Map­box tile lay­ers

Added a user in­ter­face for cre­at­ing and man­ag­ing Map­box GL of­fline tile packs. This cur­rently has a very ba­sic user in­ter­face, but it al­lows Map­box-pro­vided basemaps to be used ef­fec­tively in the field.

Bug­fixes

• Fixed a bug with the con­trol panel for cre­at­ing tile lay­ers from ex­ter­nal URLs
• Fixed some mi­nor dis­play bugs