add 16, 24, and 128 px icon sizes

[kmoad]

Flathub builds are failing due to lack of icon sizes, this fixes the build so that flathub can update to latest version.

https://github.com/flathub/com.github.artemanufrij.regextester/pull/3

[Cogitri]

Missing following diff:

diff --git a/data/icons/meson.build b/data/icons/meson.build
index 640d263..926e90d 100644
--- a/data/icons/meson.build
+++ b/data/icons/meson.build
@@ -1,4 +1,4 @@
-icon_sizes = ['32', '48', '64']
+icon_sizes = ['16', '24', '32', '48', '64', '128']
 
 foreach i : icon_sizes
   install_data(