How can I customize the toolbar (left or top)?
which toolbar? the on the left? right clicking on the top most button will pop a menu with the allowed positions. the palette bar and top tool bar can't be moved as far as I know.
I miss an "Undo" or "crop" icon
there is the menu image\crop to selection (ctrl+alt+x) that I use from keyboard but yeah an undo/redo and crop button in one of the toolbars would be nice.