feat(ui/shared): allow ModalPopup to receive header components

Similar to its `footer` content, ModalPopup can now receive header components
allowing consumers to design the header part of the modal to their likings.

Usage:

```
ModalPopup {
    id: popup

    header: Item {
			...
		}
	  ...
}
```

The header section will take the height of the root element of the substree
passed to `header`

ui/app/AppLayouts/Chat/components/ProfilePopup.qml

@@ -7,7 +7,16 @@ import "./"
 
 ModalPopup {
     id: popup
-    title: qsTr("User profile")
+    header: Text {
+      text: qsTr("User profile")
+      anchors.top: parent.top
+      anchors.left: parent.left
+      font.bold: true
+      font.pixelSize: 17
+      anchors.leftMargin: 16
+      anchors.topMargin: Theme.padding
+      anchors.bottomMargin: Theme.padding
+    }
 
     Rectangle {
       id: profilePic

ui/app/AppLayouts/Chat/components/PublicChatPopup.qml

@@ -9,7 +9,7 @@ import "./"
 ModalPopup {
     id: popup
     title: qsTr("Join public chat")
-    
+
     onOpened: {
         channelName.text = "";
         channelName.forceActiveFocus(Qt.MouseFocusReason)

ui/app/AppLayouts/Wallet/Components/AddAccountWithSeed.qml

@@ -6,7 +6,6 @@ import "../../../../shared"
 
 ModalPopup {
     id: popup
-    title: qsTr("Add account with a seed phrase")
     height: 600
 
     property int marginBetweenInputs: 38
@@ -17,6 +16,8 @@ ModalPopup {
         passwordInput.forceActiveFocus(Qt.MouseFocusReason)
     }
 
+    title: qsTr("Add account with a seed phrase")
+
     Input {
         id: passwordInput
         placeholderText: qsTr("Enter your password…")

ui/shared/ModalPopup.qml

@@ -5,8 +5,9 @@ import "../imports"
 import "./"
 
 Popup {
-  property string title: "Default Title"
+  property string title
   default property alias content : popupContent.children
+  property alias header: headerContent.children
 
   id: popup
   modal: true
@@ -27,15 +28,32 @@ Popup {
   }
   padding: 0
   contentItem: Item {
-      Text {
-          id: modalDialogTitle
+      
+      Item {
+        id: headerContent
+        width: parent.width
+        height: {
+          var idx = !!title ? 0 : 1
+          return children[idx] && children[idx].height + Theme.padding
+        }
+        anchors.top: parent.top
+        anchors.left: parent.left
+        anchors.right: parent.right
+        anchors.bottomMargin: Theme.padding
+        anchors.rightMargin: Theme.padding
+        anchors.leftMargin: Theme.padding
+
+        Text {
           text: title
           anchors.top: parent.top
           anchors.left: parent.left
           font.bold: true
           font.pixelSize: 17
           anchors.leftMargin: 16
-          anchors.topMargin: 16
+          anchors.topMargin: Theme.padding
+          anchors.bottomMargin: Theme.padding
+          visible: !!title
+        }
       }
 
       Rectangle {
@@ -74,14 +92,13 @@ Popup {
       
       Separator {
           id: separator
-          anchors.top: modalDialogTitle.bottom
+          anchors.top: headerContent.bottom
       }
 
       Item {
         id: popupContent
         anchors.top: separator.bottom
         anchors.topMargin: Theme.padding
-        anchors.bottom: separator2.top
         anchors.bottomMargin: Theme.padding
         anchors.left: parent.left
         anchors.leftMargin: Theme.padding
@@ -97,6 +114,7 @@ Popup {
 
       Item {
         id: footerContent
+        height: children[0] && children[0].height
         width: parent.width
         anchors.top: separator2.bottom
         anchors.left: parent.left