Remove the 1 pixel line on the top when hiding the tabs

[rodarima]

It seems to be caused by resizing the tab bar to 1 pixel of height:

dillo/src/uicmd.cc

Lines 363 to 366 in 6d5b3ee

	if (num_tabs() == 1) {
	// hide tabbar
	ctab_h = 1;
	CloseBtn->hide();

Setting it to 0 doesn't work well.