New function: When previewing a work, press the shortcut key C to download the current picture (Patreon)

Previously, when previewing a work, press the shortcut key `D` to download the work.

But a work may have multiple pictures, sometimes users only want to download the previewed picture (for example: the second picture of this work), instead of downloading all the pictures in the work.

Since version 15.4.0, you can press the shortcut key `C` to download only the currently displayed image.