Add proposed & cherry-picked patches

* ui-Theme-lookup-should-respect-XDG_DATA_DIRS.patch:
  - Renamed from ubuntu_theme_lookup_xdg.patch and cherry-picked from
    upstream
* StIcon-only-compute-shadow-pipeline-when-the-texture-is-p.patch:
  - Don't try to compute shadows on not allocated icons (LP: #1723378)
* js-fix-invalid-access-errors.patch:
  - Fix javascript errors (LP: #1747566)
* popupMenu-Fix-wrong-call-to-clutter_actor_add_child.patch:
  - Cherry-pick from upstream
* shell-ignore-invalid-window-monitor-index.patch:
  - Fix crash on accessing on invalid monitor windows (LP: #1724439)
* volume-Add-back-sound-feedback-on-scroll.patch:
  - Fix regression causing missing feedback on volume slider scroll
* workspace-fix-repositioned-windows-in-activities.patch:
  - Ensure windows thumbnails coordinates are correct (LP: #1653153)
* workspaceThumbnail-initialize-porthole-based-on-workArea.patch:
  workspaceThumbnail-only-update-_porthole-if-the-overview-.patch:
  workspaceThumbnail-rebuild-thumbnails-if-workareas-size-c.patch:
  - Cherry-picks from upstream, avoid unneeded computations in activities
* st-texture-cache-Cancel-sliced-image-loading-on-target-ac.patch:
  - Fix possible crash on cache loading
* st-texture-cache-Don-t-add-NULL-textures-to-cache.patch:
  - Fix crash when deleting NULL textures from cash (LP: #1754445)