specify bitmap export size in units other than pixels

[Danielmader]

The current Export Bitmap dialog lacks the very important function to
specify a certain image print size at a defined resolution in "Bitmap
Size". A drop-down menu to choose the unit measure is needed here, too.

The resulting print size should be stored in the image header, too!

[Tjvigil66 (tjvigil66)]

Originator: NO

I wouldn't say that it is needed, but it would certainly be helpful. The
size of the exported bitmap is set in your document preferences. The DPI
is set in the Inkscape preferences, in the "Misc" settings.