Generate notification icons

[ath0mas]

Add parameters for notification icons generation: --notification-icon for path, default notification.png, and --notification-icons to activate it.

Fix #144

TODO:

• init notification icon template
• manage a list of notification paths, instead of a single one
• rework parameters to be able to generate each type separately
  • main: standard-only, rounded-only, both, or none (see Add options for generating only/no rounded icons #129)
  • adaptive
  • notification