Improve how stickers appear in the emoji picker

First, the fill mode for the sticker images shouldn't stretch them.
Also make sure there is enough padding in the category so the image
doesn't appear larger than the button. Finally, set the source size for
the images so Qt can smooth them out better.

(cherry picked from commit 16f4e17e8f)

src/chatbar/EmojiDelegate.qml

@@ -43,6 +43,9 @@ QQC2.ItemDelegate {
             anchors.fill: parent
             visible: root.emoji.startsWith("mxc") || root.isImage
             source: visible ? root.emoji : ""
+            fillMode: Image.PreserveAspectFit
+            sourceSize.width: width
+            sourceSize.height: height
         }
     }
 

src/chatbar/EmojiPicker.qml

@@ -201,8 +201,13 @@ ColumnLayout {
             width: root.categoryIconSize
             height: width
             checked: stickerModel.packIndex === model.index
+            padding: Kirigami.Units.largeSpacing
+
             contentItem: Image {
                 source: model.avatarUrl
+                fillMode: Image.PreserveAspectFit
+                sourceSize.width: width
+                sourceSize.height: height
             }
             QQC2.ToolTip.text: model.name
             QQC2.ToolTip.delay: Kirigami.Units.toolTipDelay