GUI/embark-anywhere land bridging

[Pseudothrax13]

The tooltip explaining bridging gaps is not currently accurate for the steam version.

Bridging for armies in fortress mode does not currently check only the top-left tile of embarks, the entire embark area is used to check for army-access. The map is also made up of 48x48 tiles instead of 16x16, but that matters less.

To test it myself, I embarked on an isolated island. I tried to initiate a mission, to a nearby area on the mainland, but saw the inaccessible location message. I quit without saving, and then embarked to bridge the gap. Thankfully, I got both pieces of land on a single max-size embark area and didn't need to do two. I retired that one, and then embarked a second time on the isolated island. This time I was able to initiate the mission, and upon viewing the report I did confirm that they actually went to the place and came back. They also returned with a bunch of stupid stuff they didn't have when they left.

DFhack is the only mod that I use, and my only additions to the base DFhack code are a small rewrite of the item script and an OnMapLoad script I made. Nothing that should reasonably be impacting this stuff. I can reproduce the land bridging, this time taking screenshots if desired.