Removed "mutable" specifier from variables in Sheet class. All computations
are now done in execute(), i. e after recompute is invoked.
Added PropertySheet::Signaller variable to importFile method, to emit
aboutToSetValue/hasSetValue correctly.
Removed obsolete code.
Moved createRectangles() to Utils.h/cpp files.
Added call to Sheet::execute() to update model properly.
Moved Range class to separate file. Added using namespace ...
Redesigned Cell properties dialog to use Tabs.
Added support for Cell aliases.
Re-added SpreadsheetView::keyPressEvent to make Delete key work again.
Fixed Expression::toString() to generate a valid string when units are used.
Removed CmdSpreadsheetController class.
Made VariableExpression::getProperty() const.
Removed PropertyController traces.
Various fixed related to exceptions and dependencies.