Commit b1cc87cb ("msi: The line control has a height of exactly 2
device units.", 2007-12-16) open-coded the body of
(msi_)dialog_add_control() inside dialog_line_control() modulo the
height override.

Except the fixed height, line controls have nothing special compared to
other controls.  Thus, there is little merit in open-coding it.

Also, dialog_line_control() has bit-rotted over time; it already missed
a few changes that were done to any other controls.

Fix this by just using dialog_add_control().  Also, add a special case
logic just for line controls.