If I eneable "Left mouse button pans when Space is presssed" in Settings->Scrolling I have the following behaviour: If I hold space key and right mouse button I can move current view by dragging mouse. How can I modify this shourtcut to not have to hold mouse button? Just like in Gimp - hold space key and drag mouse to move the current view. Probably this could be configured in ~/.inkscape/keys/default.xml file, but I have no idea what should I put there.

You're in good company.

I don't think you can do this with Inkscape (I could be wrong though). The current behaviour mimicks Illustrator (and other similiar products). You might like to check the Launchpad site to see if it has already been requested and if it hasn't log a new bug for it.