Improve the connection tab

[GriffinRichards]

• There should be a list dropdown for selecting a connection. Currently if a connection is not visible, and not first/last in the list, it cannot be selected. This can happen if connections overlap (and it could also happen if a connection is moved offscreen; this is the only reason moving them offscreen is disallowed)

• Connecting maps to themselves is not well-supported:

  • If mirroring is on, connecting a map to itself does not show the mirrored connection until the map is reloaded.
  • If mirroring is off, connecting a map to itself does not show the connection on the opposite side (which would appear in-game) (this isn't exactly reflective of in-game behavior, the connection will only appear if loaded via the connected map)
  • If a map has a mirrored connection to itself and the connection's position is changed the mirrored side is not updated until the map is reloaded.
  • Porymap can crash if a map has more than one connection to itself on the same side and one is deleted.
    Connecting maps to themselves is sort of niche, but the crash at least should be fixed.

• Transparent map border appears above connections after switching maps (51b57f9)

• Creating a mirrored connection does not mark the connecting map with unsaved changes

• Connections are not hidden in time when dragging them around
  

• Connections with unknown direction names will lock their position in the upper right corner. They should log a warning and not be displayed.

• Calling utility.setBorderVisibility while on the Connections tab can cause the border to render incorrectly (additionally, it never updates the Border checkbox, regardless of which tab is active when it's called)

• The state of the Mirror to Connecting Maps check box is not saved (should it be? and if so would it be project-specific?)