Fix image URL preview, add Thumbnail.create()

- Make HMxcImage's mxc property work with http too (temporary quick
solution)

- Thumbnail objects can now be initialized with existing bytes and not
  have to download anything.

src/qml/Base/HMxcImage.qml

@@ -31,10 +31,18 @@ HImage {
 
         if (! image) return  // if it was destroyed
 
-        py.callClientCoro(clientUserId, "media_cache.thumbnail", arg, path => {
-            if (! image) return
-            image.cachedPath = path
-            show             = image.visible
-        })
+        if (! image.mxc.startsWith("mxc://")) {
+            source = mxc
+            show   = image.visible
+            return
+        }
+
+        py.callClientCoro(
+            clientUserId, "media_cache.get_thumbnail", [mxc, w, h], path => {
+                if (! image) return
+                image.cachedPath = path
+                show             = image.visible
+            }
+        )
     }
 }

src/qml/Chat/Timeline/EventImage.qml

@@ -2,17 +2,19 @@ import QtQuick 2.12
 import "../../Base"
 import "../../utils.js" as Utils
 
-HImage {
+HMxcImage {
     id: image
     horizontalAlignment: Image.AlignLeft
-    sourceSize.width: theme.chat.message.thumbnailWidth  // FIXME
-    source: animated ? openUrl : loader.previewUrl
+    sourceSize.width: 640  // FIXME
+    sourceSize.height: 480  // FIXME
     animated: loader.singleMediaInfo.media_mime === "image/gif" ||
               Utils.urlExtension(loader.mediaUrl) === "gif"
+    clientUserId: chatPage.userId
+    mxc: animated ? openUrl : (loader.thumbnailMxc || loader.mediaUrl)
 
 
     property EventMediaLoader loader
-    readonly property url openUrl: loader.mediaUrl || loader.previewUrl
+    readonly property url openUrl: loader.mediaUrl || loader.thumbnailMxc
 
 
     TapHandler {

src/qml/Chat/Timeline/EventMediaLoader.qml

@@ -8,7 +8,7 @@ HLoader {
 
 
     property QtObject singleMediaInfo
-    property url mediaUrl
+    property string mediaUrl
     property string showSender: ""
     property string showDate: ""
 
@@ -47,21 +47,17 @@ HLoader {
         return EventDelegate.Media.Page
     }
 
-    readonly property url previewUrl: (
-        type === EventDelegate.Media.File ||
-        type === EventDelegate.Media.Image ?
-        singleMediaInfo.thumbnail_url : ""
-    ) || mediaUrl
+    readonly property string thumbnailMxc: singleMediaInfo.thumbnail_url
 
 
-    onPreviewUrlChanged: {
+    onTypeChanged: {
         if (type === EventDelegate.Media.Image) {
-            var file  = "EventImage.qml"
+            var file = "EventImage.qml"
 
         // } else if (type === EventDelegate.Media.File) {
         //     var file  = "EventFile.qml"
         //     var props = {
-        //         thumbnailUrl: previewUrl,
+        //         thumbnailMxc: thumbnailMxc,
         //         fileUrl:      mediaUrl,
         //         fileTitle:    info.media_title,
         //         fileSize:     info.media_size,