feat(database): implement function to delete rows from db tables

lib/transport_helper.dart

@@ -130,6 +130,38 @@ class Route {
       );
     }
   }
+
+  void dbDelete() async {
+    Database database = DbHelper.db;
+    int result = await database.delete(
+      "route",
+      where: "id = ?",
+      whereArgs: [id],
+    );
+    assert(result == 0);
+  }
+
+  void dbDeleteLastRouteSegment() {
+    Database database = DbHelper.db;
+    database.delete(
+      "routesegments",
+      where: "route = ? AND segmentorder = ?",
+      whereArgs: [id, segments.length - 1], // TODO: test the index
+    );
+  }
+
+  void dbDeleteTrailingRouteSegment(int position) {
+    Database database = DbHelper.db;
+    if (position <= segments.length) {
+      database.delete(
+        "routesegments",
+        where: "route = ? AND segmentorder >= ?",
+        whereArgs: [id, position],
+      );
+    }
+  }
+
+  // TODO: delete leading segments and decrement following segments
 }
 
 class Segment {
@@ -196,6 +228,15 @@ class Segment {
     Database database = DbHelper.db;
     id = await database.insert(tableName, toMap());
   }
+
+  void dbDelete() {
+    Database database = DbHelper.db;
+    database.delete(
+      tableName,
+      where: "rowid = ?",
+      whereArgs: [id],
+    );
+  }
 }
 
 class Station {
@@ -266,6 +307,17 @@ class Station {
       database.insert("stationlines", stationLines[i]);
     }
   }
+
+  void dbDelete() async {
+    Database database = DbHelper.db;
+    database.delete(tableName, where: "stationid = ?", whereArgs: [id]);
+  }
+
+  /// remove all Lines connected to a Station - should be used together with some kind of reparsing.
+  void dbDeleteStationLines() async {
+    Database database = DbHelper.db;
+    database.delete("stationlines", where: "stationid = ?", whereArgs: [id]);
+  }
 }
 
 class Vehicle {
@@ -308,6 +360,11 @@ class Vehicle {
     Database database = DbHelper.db;
     database.insert(tableName, toMap());
   }
+
+  void dbDelete() async {
+    Database database = DbHelper.db;
+    database.delete(tableName, where: "id = ?", whereArgs: [id]);
+  }
 }
 
 class Line {
@@ -378,6 +435,11 @@ class Line {
     Database database = DbHelper.db;
     database.insert(tableName, toMap());
   }
+
+  void dbDelete() async {
+    Database database = DbHelper.db;
+    database.delete(tableName, where: "lineid = ?", whereArgs: [lineId]);
+  }
 }
 
 // TODO: add fromJson
@@ -424,6 +486,11 @@ class Departure {
     Database database = DbHelper.db;
     id = await database.insert(tableName, toMap());
   }
+
+  void dbDelete() async {
+    Database database = DbHelper.db;
+    database.delete(tableName, where: "id = ?", whereArgs: [id]);
+  }
 }
 
 class Color {