ui: Update check icon

Use the standard check icon, which is less heavy.

data/resources/icons/scalable/actions/check-plain-symbolic.svg

@@ -0,0 +1,2 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" height="16px" viewBox="0 0 16 16" width="16px"><path d="m 13.753906 4.65625 c 0.175782 -0.199219 0.261719 -0.460938 0.246094 -0.722656 c -0.019531 -0.265625 -0.140625 -0.511719 -0.339844 -0.6875 c -0.199218 -0.175782 -0.460937 -0.265625 -0.726562 -0.246094 c -0.265625 0.015625 -0.511719 0.140625 -0.6875 0.339844 l -6.296875 7.195312 l -2.242188 -2.246094 c -0.390625 -0.390624 -1.023437 -0.390624 -1.414062 0 c -0.1875 0.1875 -0.292969 0.445313 -0.292969 0.710938 s 0.105469 0.519531 0.292969 0.707031 l 3 3 c 0.195312 0.195313 0.464843 0.300781 0.742187 0.292969 c 0.277344 -0.011719 0.535156 -0.132812 0.71875 -0.34375 z m 0 0" fill="#222222"/></svg>

data/resources/icons/scalable/actions/checkmark-symbolic.svg

@@ -1,2 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<svg xmlns="http://www.w3.org/2000/svg" height="16px" viewBox="0 0 16 16" width="16px"><path d="m 13.398438 2.011719 c -0.394532 0.027343 -0.765626 0.210937 -1.027344 0.507812 l -5.78125 6.609375 l -1.890625 -2.519531 c -0.496094 -0.664063 -1.4375 -0.796875 -2.101563 -0.300781 c -0.316406 0.238281 -0.527344 0.59375 -0.582031 0.988281 c -0.058594 0.394531 0.046875 0.796875 0.285156 1.113281 l 3 4 c 0.273438 0.359375 0.691407 0.582032 1.144531 0.597656 c 0.449219 0.015626 0.886719 -0.171874 1.183594 -0.511718 l 7 -8 c 0.542969 -0.625 0.480469 -1.570313 -0.140625 -2.117188 c -0.300781 -0.261718 -0.691406 -0.394531 -1.089843 -0.367187 z m 0 0" fill="#222222"/></svg>