Merging vertices and clearing degenerate linedefs fails while dragging linedef

[ioan-chera]

Let's say we have this wall:

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|
|
|
x-------x3
        |
2       |1
        |
        |
x-------x4
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|
|
|

Dragging line 1 to the column occupied by number 2 will not merge vertices 3 and 4 as expected. The misbehaviour gets worse if there are multiple sectors involved.

This is altogether a missing feature. Currently Eureka only performs splitting or merging when dragging a single vertex. Nothing happens when dragging multiple vertices or anything like linedefs or sectors. Since it's a very common operation and we never want broken layouts, the behaviour needs to be consistent in all modes. So here's a check list:

• [x] Merge vertices when dragging other objects than single vertices
• [x] Split lines when dragging other objects than single vertices
• [x] Apply the correct linedef sandwich merge when linedefs get merged
• [ ] #145
• [ ] #146
• [ ] #147
• [x] Show appropriate status messages