gui1: Reduce menu column text padding to 2px, use only on headers
gui2/tprogress_bar: Use same background as listbox (children) widgets

Replaces the solid RGB background with the generic RGBA background
defined in GUI__BACKGROUND_COLOR_ENABLED.
gui2/tchat_log: Use GUI_FORCE_WIDGET_MINIMUM_SIZE macro instead of local hack

Turns out there already was a somewhat more elegant solution in use for
the GUI2 lobby and macroified for everyone else. It uses a
stacked_widget with a spacer within instead of the awkward 2x2
{[central widget,spacer],[spacer,spacer]} grid we were using here.