Improve "favourite servers" feature

The commit:
1. does serverdescription keyword case insensitive for
showonlyfavourites > 0;
2. fixes typo in argument of alias in keyword;
3. fixes disabling background color for servers of the category, when
alpha is changed to 0;
4. shows 0 instead of 10 in info about digit, which toggles 10th category;
5. adds info about "image" attribute in favourites.cfg;
6. adds new feature: "Search for servers".