proposal inline pager

This adds an additional row type for proposals. That row already contains important parts of the proposal like description, a vote widget and comments in an overlay.

Note that atm the row is not used anywhere. To test this, you need to add `inline=True` to a proposal pager.

There is currently no UI for selecting this alternate row because I could not yet think of an option that is simple and yet generic.

This was developed for cases in which category pages are enabled while the proposal tab is hidden. I guess it can be used with milestones in the same way.