This adds menu item icons for Add Mask, Flatten Image, Fit Image To
View, and Generic 5x5 Convolution.
This modifies the menu item icon for Swap Colors to make the action more
obvious and improve accessibility.
Each of these strings would previously rely on StringView's char const*
constructor overload, which would call __builtin_strlen on the string.
Since we now have operator ""sv, we can replace these with much simpler
versions. This opens the door to being able to remove
StringView(char const*).
No functional changes.
