rename icons

Rename icon files to solve CMake (ECM) warnings.

```
CMake Warning (dev) at /usr/share/ECM/modules/ECMInstallIcons.cmake:248 (message):
  "hi22-apps-muon.png" has characters before the size; it should be renamed
  to "22-apps-muon.png"
Call Stack (most recent call first):
  src/icons/CMakeLists.txt:1 (ecm_install_icons)
```

(if accepted, please push it for me)

REVIEW: 126710