FlxTrail bugfixes and changes

- Made `solid` public like it should be
- Removed reference to `_solid` which doesn't exist anymore
- Only change the value of `solid` of newly created `trailSprite`s

This version of `FlxTrail` should probably also be merged to the flixelNME branch soon, as the version there is bugged and doesn't compile. This version has been tested and compiled by me and doesn't seem to contain any errors.