List of features to be implemented (not in any particular order)
----------------------------------------------------------------
+- de-map space key from button bar
- lasso selection tool
- prefs file, save prefs
- flatten (incl undo/redo...) - enabled only if nlayers>1
- recalibration upon screen resize / compensation for miscalibration
- find a better behavior for vertical space tool across page boundaries ?
- key mappings (pgup/dn for full page, up/dn for scroll)
+- zoom increments more flexible (1% instead of 5% ?)
+- copy/paste of an entire page (beware if PDF bg is not compatible!)