[UX] Use a full-width plus for buttons paired with a unicode minus (#7018)

U+FF0B vs. U+002B

Because otherwise it is not, in fact, the same width as the minus, which looks weird in buttons ;).

Re #3840 & #6939

frontend/ui/widget/buttonprogresswidget.lua

@@ -180,7 +180,7 @@ function ButtonProgressWidget:update()
         local margin = button_margin
         local extra_border_size = 0
         local button = Button:new{
-            text = "+",
+            text = "＋",
             radius = 0,
             margin = margin,
             padding = button_padding,