tweaking of world/mod menu

- Add help screen and allow moving between the tabs by pressing TAB (fix #5975);
   Anyone who can/want to expand the description in help screen is welcome. Function of items blacklist can be added here.
- Remove name of the mod from the mod description window (this is duplication, name is presented in the lists above);
- Move tips about shortcuts out of the description window;
- Make captions of mod properties (Name, Author, Description, Dependencies) translatable;
- Remove extra 'continue', see a line note (is this okay, BTW?).

Screenshot of updated window:
![mod](https://f.cloud.github.com/assets/5421954/2106818/31590038-8fb4-11e3-8949-79f871ff0bfa.png)


P. S. I accidentally discovered file mod_manager_ui.cpp and it looks like a code duplication, for ui lines, at least.