Adjust icons names to better fit FDO scheme

An icon theme may use variously styled icons for various types of UI
elements. For example, Breeze theme uses monochrome icons for actions
(toolbars) and statuses, but colourful for view contents. qBittorrent
was using icon names without putting attention to categories, creating
mix of styles in UI elements (i.e. the main toolbar).

This commit fixes icon names (and move some of them to theme) to avoid
this style mixes where possible.

Unfortunately, options dialog page icons for network and speed limits
are remained monochrome because there are no Breeze or Adwaita icons to
match.

src/icons/qbt-theme/build-icons/icons/folder-download.svg

@@ -0,0 +1,16 @@
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