fix #832, floating btns closing, Esc to close confirmation dialog, 0x0 rects disabled

[olekhshch]

PR description

-- Esc key cancels the dialog window appearing before creating a new document (N) or opening new svg file (atm Esc key acts as Enter and selects currently focused button (usually it's an "Ok" choice) which is a bit confusing);

-- Floating buttons (e.g. Shape Library or Rect/Square/Rect by hand) can be closed by clicking somewhere in the document (shapeLibrary - by clicking on the workarea and button itself, not only on the handle);

-- ShapeLibrary can be cancelled with Esc key (same as here);

-- Drawing of 0x0 rectangles/squares is disabled (atm a simple click while in rect or square creates such figure)

-- Addressing #832 - the mode will not change to select if Alt was pressed. I've chosen this key because Shift is reserved for Circle/Square while drawing Ellipses/Rect.

-- after drawing a shape from the Shape Library (if Alt is not pressed) the mode changes to 'select' (same as with other tools, atm after drawing a shape mode doesn't change automatically)

[jfhenon]

Thanks