Put icons in their own custom theme

You can't view the tokodon-chat-reply icon without installing it to your system, so I just went ahead and moved all of our icons to their own custom themes. We have tokodon (light and colorable icons) and tokodon-dark (dark versions of uncolorable icons) sitting in :/icons. I tried shoving them under :/content/icons and setting the theme search path - Qt did not like it :S

The one bad thing is that there's no good way to tell if the current icon theme is "dark" except for searching for "dark" in the theme name. It falls back to light icons currently.