Use collapse(2) when not building with gcc 4.8.1

See bug #9456.
masks: fix braindead bug that has caused an excessive number of nodes for brush strokes
masks: on deletion of brush nodes: remove complete shape if only one node is left
Silence calculator error messages

People didn't appreciate these:
error: T
error: R
error: U
error: E
exiv: parse FocusDistanceUpper and FocusDistanceLower (specific for Canon) (needs exiv2-0.25)
masks: some further improvements for brush node elimination
Usermanual: mention slideshow view and overwrite on export