Cosmetic changes to improve the layout of the preferences dialog.

I've fiddled around some more with the layout of the startup prefs. I made the prefs window slightly larger (350x470), put the startup flags field on a separate line so that it lines up with the paths/libs tables, and made all the elements expand with the size of the prefs window, so that the user can just resize the dialog to get a larger view of the tables. It's not perfect yet (I can't seem to bump the table sizes up beyond the "magic numbers" of height:24vh; width:89vw; without getting scroll bars at 350x470), but it's the best that I could do, and in any case I think that it looks a lot nicer than before. The other tabs still look all right to me as well. Tested on both Linux and Mac, see the screenshots below. Couldn't test on Windows, so it would be nice if you could give it a whirl there and see whether it looks ok there, too, and merge if you like it.

Linux:

Mac:

NB: I've also removed the height: 4vh value from the tab labels in the css files (rev. e75869f2). This made the labels expand vertically when increasing the window size, which looks rather bad, as can be seen in the screenie below: