Some code refactoring:
* use GUI_App::open_preferences() on all places where it's needed
* Preferences Dialog is an attribute of a ManeFrame class and created just ones during the MainFrame creation now.
* Created class Highlighter. Use it in Preferences and Tab
changes in hints.ini
Hint notification: Just one button in Help menu.
Hint notification: New naming - Tip of the day
Notifications: no scrolling.
Hint notification: fadeout colors.
Hints notification: enabled / disabled tags
Hints notification: suppress opening url due to preferences.
Hint notification: close after 5 minutes and button in Help menu
Hint notification bold (diff color) headline on multiple lines
Hint notification: Documentation link button.
Hints.ini header
hints logo - clippy
disabled a preffered modes and techs for hints notifications
Empty hints array check. Fix of #6774.