[is Crop to selection possible]

This option is there, pretty much. 
Select what you want, then file>export and chose export selection under 'area'.

If you want to crop a layer, make the selection you want to crop to, press Ctrl+shift+i (to select inverse) then press delete.

 

Also note you can save the selected area so that you only have to make a precise selection once. Go to select>save selection.