Labels in headers reserved space for an icon even when no icon was used.
This is caused by the shared function ui_text_icon_width adding 1.5x
a buttons X-units width the the width of the string.
Menu buttons detected this and subtracted the extra padding.
Instead of adding the same workaround for labels,
add ui_text_icon_width_ex that takes a padding argument.
Add presets for 'default', 'compact' and 'none' to avoid duplicating
padding values.
This allows removal of hard-coded label scaling for the add-object tool.
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