sender-avatar: Add a little brightness effect to avatar on hover

To make it clearer that you can interact with it.

data/resources/style.css

@@ -777,14 +777,26 @@ sender-avatar:hover {
   background-color: alpha(currentColor, .07);
 }
 
+sender-avatar:hover image {
+  filter: brightness(1.07) ;
+}
+
 sender-avatar:active {
   background-color: alpha(currentColor, .16);
 }
 
+sender-avatar:hover image {
+  filter: brightness(1.16) ;
+}
+
 sender-avatar:checked {
   background-color: alpha(currentColor, .1);
 }
 
+sender-avatar:hover image {
+  filter: brightness(1.1) ;
+}
+
 sender-avatar popover button.text-button {
   padding-left: 10px;
   padding-right: 10px;