fix(editor): a slip goes to the crossing under the cursor or nowhere, shown before the click

A slip went to the crossing of the selected straight with another straight within 40 m of the click. When the track under the cursor wasn't taken as a straight (a slight curve, a switch), the slip silently went to another crossing up to 40 m away. A double slip within 25 m of the click was replaced, even when the click was on a free crossing.

- The crossing nearest the cursor, within 12 m, is found among all the rail tracks. A slip goes only there; when that track is a curve or a switch, the tool says so.
- A double slip is replaced only when it is nearer the cursor than any crossing.
- The cursor is taken at the height of the straight under it, so on a grade the 3D view doesn't put the click metres away from the track.
- Before the click, the view marks the crossing the slip goes to, with its two straights, or the reason it can't go there.

application/editormodetrackshared.h

@@ -103,6 +103,7 @@ float const kHandleRadius{10.0f};
 double const kLaySnapRadius{5.0};
 float const kSnapPixels{16.0f};
 double const kSwitchReach{6.0}; // a press farther from the selected track selects another one instead of placing the switch
+double const kSlipReach{12.0}; // m; a slip goes to the crossing nearest the cursor, if it's this close
 double const kLineReach{2000.0}; // how far each way the Curve tab follows the line
 double const kDigitizeFit{0.5};
 double const kDrawnReach{100000.0};