Option to add dummy rectangle matching symbol bounding box

[Pander]

When adding symbols from Ctrl+Shift+Y, he symbols are missing a bounding box. This bounding box is not needed for the actual symbol itself, but one is essential for aligning symbols according to their original positions in the bounding box of their original design.

An exmaple of how this could be solved is shown in https://i.imgur.com/jluS16l.png where a symbol is placed grouped with a pink bounding box. In this way, symbols can easily be resized (with locked aspect ration) and aligned to other elements in a proper way.

[Pander (pander)]

See also https://bugs.launchpad.net/inkscape/+bug/1464270