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/utils.js

@@ -154,25 +154,6 @@ function filterModelSource(source, filter_text, property="filter_string") {
 }
 
 
-function thumbnailParametersFor(width, height) {
-    // https://matrix.org/docs/spec/client_server/latest#thumbnails
-
-    if (width > 640 || height > 480)
-        return {width: 800, height: 600, fillMode: Image.PreserveAspectFit}
-
-    if (width > 320 || height > 240)
-        return {width: 640, height: 480, fillMode: Image.PreserveAspectFit}
-
-    if (width >  96 || height >  96)
-        return {width: 320, height: 240, fillMode: Image.PreserveAspectFit}
-
-    if (width >  32 || height >  32)
-        return {width: 96, height: 96, fillMode: Image.PreserveAspectCrop}
-
-    return {width: 32, height: 32, fillMode: Image.PreserveAspectCrop}
-}
-
-
 function fitSize(width, height, max) {
     if (width >= height) {
         let new_width = Math.min(width, max)