Make agui::Filler available

[dockmeister]

Hi,

i would like to stay consistent with the vanilla gui style.

There are a few cases where the default filler within a frame caption is not enough:
- adding an On/Off radiogroup inside a frame header (like the constant combinator gui)
- separating two buttons by a stretchable filler (like the load savegame gui)

cheers

edit: sorry, this request already exist here: 66580