Implement category search in library

[stefaanternier]

First this screen must be shown.

This screen can be generated by the following web service:
http://streetlearn.appspot.com/network.html?path=store/categories/lang/nl
http://streetlearn.appspot.com/network.html?path=store/categories/lang/en
Next, given a category, you need to show

This information can be retrieved using:
http://streetlearn.appspot.com/network.html?path=store/games/category/1
where "1" is the category id for "culture/cultuur"

[wvdvegt]

1. Started with implementing this (a first working version is checked in).
2. Not very handy is that store/games/category/1 returns a GameCategoryList instead of a GameList (which contains all info to render the items). It means making 1-2 calls extra for each new game encountered to get title and icon.