Removed Cell::setExpression(), replaced usage bu setContent().
Added ExpressionVisitor class, to be able to visit and possibly modify
each node of an expression.
Added code to rewrite expressions when rows/columns are inserted or removed.
Update the cmake files
Fix compiling on windows

Tested with VS 2012
Fixed crash in certain cases.
Fixed typo.
Fixed icon for Spreadsheet tab.
Improved error reporting.
Replaced Q_ASSERT by assert in PropertySheet.
Fixed PropertySheet::cellAt to always return anchor cell (i.e upper left
cell of merged cells).
Replaced freeze/unfreeze concept by Signaller class, to invoke
aboutToSetValue() and hasSetValue() appropriately.
Removed printf and old method definitions.
Added assert.
Refocus model when cells are merged.
Removed hasSetValue() calls (done by Signaller class now).
Moved dependency tracking into PropertySheet class.
Added menu.