fix(settings): reduce keyboard memory usage

test/ui_layout.lua

@@ -20,6 +20,13 @@ assert(rect(title) == "12,12 296x8", rect(title))
 assert(rect(first) == "12,28 296x24", rect(first))
 assert(rect(second) == "12,60 296x24", rect(second))
 
+screen:draw()
+device.painted = {}
+first:setText("a longer label")
+screen:draw()
+assert(#device.painted == 1 and device.painted[1].label == "a longer label",
+       "changing a button label repainted other components")
+
 -- Fractions resolve against the parent content box, absolutes stay absolute.
 local half = ui.box{w = 0.5, h = 40}
 local fixed = ui.box{w = 100, h = 40}