Merge down -> check if another TRWLayer below (HOW)
Actual merge operation:
Take one layer, go thru each waypoint/track of another.
Take old name and value pointers, add to new layer, steal from old layer.
Name conflict:

 ______________________________________________________
|_____n_a_m_e_____________c_o_n_f_l_i_c_t______________|
|                                                      |
| The track 'x' exists in both the top layer ('l1')    |
| and the bottom layer ('l2'). What do you want to do? |
|                                                      |
| Delete From  :                   top layer ('l1')    |
| Rename to                                            |
| [        ] in:                                       |
| Autorename                                           |
| Delete All                       bottom layer ('l2') |
| Autorename all                                       |
+------------------------------------------------------+

TODO
* check if another TRWLayer below (HOW)
* Better/less confusing UI

UI (name conflict) suggestions would be appreciated!
