Improve look of the HoverLinkIndicator and other refactorings

It still looks generally the same, except now:
* It doesn't take up the full width of the window (this was a mistake.)
* It now nicely fades in and out.
* It uses Kirigami's built-in OverlayZStacking object instead of
hardcoding a Z of 20.
* A border is now added which helps make it stand out compared to the
chat bar, which it frequently is on top of.
* The seemingly unused accessibility string is removed.

src/app/qml/HoverLinkIndicator.qml

@@ -3,29 +3,55 @@
 // SPDX-License-Identifier: LGPL-2.0-or-later
 
 import QtQuick
+import QtQuick.Layouts
 import QtQuick.Controls as QQC2
 
 import org.kde.kirigami as Kirigami
 
-QQC2.Control {
+RowLayout {
     id: root
 
     property string text
 
-    visible: !root.text.startsWith("https://matrix.to/") && root.text.length > 0
-    Kirigami.Theme.colorSet: Kirigami.Theme.View
-
-    z: 20
-
-    Accessible.ignored: true
-
-    contentItem: QQC2.Label {
-        text: root.text.startsWith("https://matrix.to/") ? "" : root.text
-        elide: Text.ElideRight
-        Accessible.description: i18nc("@info screenreader", "The currently selected link")
+    onTextChanged: {
+        // This is done so the text doesn't disappear for a split second while in the opacity transition
+        if (root.text.length > 0) {
+            urlLabel.text = root.text
+        }
     }
 
-    background: Rectangle {
-        color: Kirigami.Theme.backgroundColor
+    Kirigami.OverlayZStacking.layer: Kirigami.OverlayZStacking.ToolTip
+    z: Kirigami.OverlayZStacking.z
+    spacing: 0
+
+    opacity: (!root.text.startsWith("https://matrix.to/") && root.text.length > 0) ? 1 : 0
+    visible: opacity > 0
+
+    Behavior on opacity {
+        OpacityAnimator {
+            duration: Kirigami.Units.shortDuration
+            easing.type: Easing.InOutQuad
+        }
+    }
+
+    QQC2.Control {
+        Kirigami.Theme.colorSet: Kirigami.Theme.View
+
+        Accessible.ignored: true
+
+        contentItem: QQC2.Label {
+            id: urlLabel
+
+            elide: Text.ElideRight
+        }
+
+        background: Kirigami.ShadowedRectangle {
+            corners.topRightRadius: Kirigami.Units.cornerRadius
+            color: Kirigami.Theme.backgroundColor
+            border {
+                color: Kirigami.ColorUtils.linearInterpolation(Kirigami.Theme.backgroundColor, Kirigami.Theme.textColor, Kirigami.Theme.frameContrast)
+                width: 1
+            }
+        }
     }
 }