Loading the thumbnail_full in preview

[ronitjadhav]

Using the thumbnail_full to load the preview image.

Some thumbnail_full resources are not downloaded properly: eg: QGIS Dashboard
And the thumbnails are not properly aligned in the center

[ronitjadhav]

@ismailsunni
I made updates to load thumbnail if thumbnail_full is not able to load, and aligning the thumbnail in center.
I noticed there are no thumbnails for Models.

[ismailsunni]

I tested it, and it's working fine and the preview looks better.
Unfortunately, the performance is slightly down for some resources with big file-size thumbnails (slower to show the preview).

Some options:

1. Add a loading icon when the full thumbnail is not fully downloaded
2. Download the full thumbnail in the background
3. Add an option in the setting to select which thumbnail to be shown (full or cropped)

1 and 2 are similar but more complicated and need #44. So, let's implement the 3rd one.

Note: I got a QGIS crash when I played with it, not sure with which resource though.