Implement Tips of the Day as a way to document/showcase features

[christophe-calmejane]

This feature would display a (rather) small floating windows (anchored to the bottom right of the matrix?) containing:

• A title
• A description
• An image to illustrate the tip of the day
• A close button
• A show Tips of the Day checkbox that disables further tips when unchecked
• An engine to randomly select a tip that was not previously displayed to the user (save already displayed tips in config file), only once a day. Each tip should be identified by a unique number in a standalone QRC file.

A new checkbox in the options to enable/disable tips, enabled by default.