add common icon, shamelessly taken from gnome's icon
implement icons installation and really use it in code
update icon cache on installation
the code is taken from geary
add .gitignore
Merge branch 'wip/descriptive-icons'

- use icon foreach device type, which is smartphone-symbolic, tablet-symbolic, computer-symbolic, laptop-symbolic.
- install icons to hicolor folder, icons is taken from Gnome icon theme
update icon visibility