DDMsgReader: Mouse click support for the bottom help lines in scrollable mode (thanks to help from Nelgin)

DDMsgReader: Mouse click support for the bottom help lines in scrollable mode (thanks to help from Nelgin). The version is now 1.52. I previously had mouse support implemented using mouse_getkey(), but I had defined specific click coordinates for the help lines for the help line items. Now I'm using mouse click hotspots, which requires a lot less code.