[Feature Request] Add support for specifying tooltip for categories

The categories section of Lutris is useful for organizing games based on the user bespoke preferences.

I think it would be useful to allow to users specify an optional tooltip associated with a category. That way it can act as a reminder to the user what the purpose of the category is for in the future.

The tooltip can appear when the category is hovered over in the sidebar.

For example if the user has a "beaten" games category, they can specify a tooltip detailing its purpose.

Notes on changes needed:

One implementation strategy that can be done to be implement the feature is as follows.

1. The CategorySidebarRow class can add a call to self.set_tooltip_text inside of its __init__ method. The tooltip can be passed in as an argument.
2. The EditCategoryGamesDialog class (the class that is responsible for the Dialog that pop-up when clicking the gear icon) can be updated to add another row that would allow the user to specify "Tooltip".
3. The categories table in the pga.db can be extended with a "tooltip" column.
   A migration module can be added to populate the column with an empty string