Rewrite media caching (old image provider)

- Doesn't use pyotherside's image provider feature, for more flexibility
  and simplicity
- Suitable for supporting matrix media events and more later
- Avoid a lot of duplicate files that the old cache created due to
  server not returning what we expect, mistakes in Python/QML code, etc
- Changed file structure
  (e.g. thumbnails/32x32/<mxc id> instead of
   thumbnails/<mxc id>.32.32.crop)

- Backend.wait_until_account_exist: start issuing warnings if the
  function runs for more than 10s, which means in most case a bad user
  ID was passed

- New HMxcImage QML component, used in H(User/Room)Avatar

src/qml/Base/HMxcImage.qml

@@ -0,0 +1,40 @@
+import QtQuick 2.12
+import "../utils.js" as Utils
+
+HImage {
+    id: image
+    source: sourceOverride || (show ? cachedPath : "")
+    onMxcChanged: Qt.callLater(update)
+    onWidthChanged: Qt.callLater(update)
+    onHeightChanged: Qt.callLater(update)
+    onVisibleChanged: Qt.callLater(update)
+
+
+    property string clientUserId
+    property string mxc
+    property string sourceOverride: ""
+
+    property bool show: false
+    property string cachedPath: ""
+
+
+    function update() {
+        let w = sourceSize.width || width
+        let h = sourceSize.height || height
+
+        if (! image.mxc || w < 1 || h < 1 ) {
+            show = false
+            return
+        }
+
+        let arg = [image.mxc, w, h]
+
+        if (! image) return  // if it was destroyed
+
+        py.callClientCoro(clientUserId, "media_cache.thumbnail", arg, path => {
+            if (! image) return
+            image.cachedPath = path
+            show             = image.visible
+        })
+    }
+}