Grid: Revert model view split, fixes #103.

After all it is more sane to take grid as a inherent property of a the sheet instead of splitting it.
ItemData, Schematic: Cleanup struct members, fix two compile warnings.
waf: Add gdb target hack for dead simple debugging.

This has yet to be implemented properly.
Coords: Move a useful definition to the header.
ItemData,Grid: Properly refresh the model item when snapped, not only the view item.

Create a changed cycle, as otherwise the model items would have to know about the view.
The cycle begins when the user modifies a widget (i.e. moves, rotates, ..)
a action specifc callback handler gets excuted which modifies the model item.
That model item - if actually changed (i.e. moved .25 grid units the item will
snap back into the very same position) the "changed" signal is emitted.
The sheet/view updates itself and snaps the item to a grid corner.
If a snap occured this will be treated as a (minor) movement and the cycle starts over,
until no more snapping is required. This is usually the second iteration.
sheet.c: Define check error.
sheet.c: fix compile error, forgoten var in ifdef.
merge: Fixes a crash due to usage of freed value, merges Grid and
GridItem into Grid, introduces update cycle between model and sheet/view.