0.3.7.0 Added Multi-Floor Support for maps

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This update, adds a Floor/Layer Management system so that each map can have multiple layers,rather than a separate map for every floor. While this update is relatively low impact, it lays the groundwork for the "Multi-Layer" rendering features which will be the next feature on the task list.

Map Layer Features:

• Added an "Add/Edit Floors" button to open the floor manager screen.
• Floor manager allows you to add layers above and below the primary map, and reorganize or delete them. Moving floors up and down automatically renames them if you leave the automatic floor name as part of the final name.
• Added a floor selector panel to the bottom right of the map display area. Mousing over a floor name pops up a preview of the floor.
• Added a "Use Main Map's Grid" checkbox on each floor in map-editor, letting you copy the grid from the main-floor, or customize your grid for the selected floor.

Bug Fixes:

• Updated Open-Image dialog filters to include *.Webp files by default.
• Fixed a number of issues with the Grid-Fill drawing tool. These weren't properly sent to anyone but the drawer, and would disappear after 10sec.
• Fixed a bug when undo-ing a Fill-Tool operation, where the DM undoing a "fill with fog" would end up erasing all of the fog for players (but working correctly on the DM)
• Improved the network code for sending maps, such that map images are automatically sent to lan clients, while only sent upon request to internet clients if a Steam-Cloud download fails.
• Simplified Player Steam API behaviors. Some clients would see Steam running with a full CPU core while playing "Dudes on a Map", along with problems with Steam friends locking up, and other oddities. This Steam issue seemed to be resolved in the Steam Beta Branch, but should also be resolved by this minimizing checking for API updates from Steam.