Control selectable packages

[jdrueckert]

Add setting to Settings View that enables the user to mark package types (and maybe versions) they don't want to have included in the drop-down selection, e.g. if they only want to see stable types.

Could be used to define a user role - specific default, e.g. for "I'm no techie and only want to play the game" users, only display "official" releases.

Could be used to achieve or use in combination with #262, e.g. have a list of all (historic) versions in the settings, only display top-ten and specifically marked versions in drop-down selection.

[jdrueckert]

Could be designed as a "Dual List Box" (e.g., https://ux.stackexchange.com/questions/28995/moving-elements-between-lists)