Improve screenshot-related section headings in README

* Don't put customized icons under Plasma section; it is not
  Plasma-specific
* Rename "Under Plasma 5" to "Plasmoid" to be more specific

README.md

@@ -108,13 +108,14 @@ The screenshots are not up-to-date.
 ### Qt Widgets based GUI under Openbox/Tint2 with dark Breeze theme
 ![Qt Widgets based GUI under Openbox/Tint2](/tray/resources/screenshots/tint2-dark.png?raw=true)
 
-### Under Plasma 5
+### Plasmoid (for KDE's Plasma shell)
 #### Light theme
 ![Plasmoid (light theme)](/plasmoid/resources/screenshots/plasmoid-1.png?raw=true)
 #### Dark theme
 ![Plasmoid (dark theme)](/plasmoid/resources/screenshots/plasmoid-2.png?raw=true)
-#### Customized icons
-![Plasmoid (customized icons)](/plasmoid/resources/screenshots/plasmoid-3-custom-icons.png?raw=true)
+
+### Icon customization dialog
+![Plasmoid (customized icons)](/tray/resources/screenshots/custom-icons.png?raw=true)
 
 ### Settings dialog
 ![Settings dialog](/tray/resources/screenshots/settings.png?raw=true)