Looks nice, and I agree with Skyld, you can have some buttons like where it is located now, but a menu bar would be nice, like
File, Edit, Level, Advanced, and Help? And also have tabs for levels
File:
New level
Open level
Save level
Save level as...
Edit:
Edit level as text
Edit signs
Edit links
Level:
Add new npc (would place at (0,0), default, or (32,32))
Add new predefined object
Add tileset (could edit TiledefsOffline.txt in tiledefs folder, or add a new script with text fitted for new tileset and mode)
Advaned:
Load all npcs (would make a gui whihc contain a gui list who list all npcs in that level, and when double clicking it would open that npc)
Help:
Could bring up a help gui showing functions and such, and explains what the buttons etc are for..
Just some ideas/suggestions, got bored
