[WIP] Enhance pinned filter so they can be removed

[Phocea]

Scope

Current pinned functionality is limitative since you define a default behavior where a filter is going to be pinned to the top of a listview, but the user has no way of removing this filter if he is not interested in it.

Being able to remoe a "pinned" filter would be great if you want to implement a way for the user to save its filter preferences, and then modify them later on!

Description

I am proposing to either:

• add a second attribute to the pinned() method of Field.js, defining if its removable or not. False by default.
• add a new method for filters, called "active", which will do the same as pinned but allowing it to be removed.

First option would make the actual form filter condition a bit more complex.

@fzaninotto , @jpetitcolas , @Kmaschta I would appreciate your opinion before to make those changes

[fzaninotto]

I don't think that having unpinnable filters without user preferences makes sense, and I don't want to take the path of user preferences (too big, and too far from the initial goal of ng-admin).

Besides, if all the pinned filters can be unpinnable, than the implementation is quite easy in userland, by overriding the maFilterForm template.

So this won't be added to the core.