Allow hover events on a link when it is disabled.

It did not work before because the link 'enabled' state reused the
base QGraphicsItem's enabled property (which affects hover events).
Expand the line item's mouse hit area.
Removed the use of QVariant.toPointF (PyQt4 api v2).
Allow signed number literals in 'tuple_eval'.
Added a separate orange icon for the canvas main window.

It used to reuse the "Get Started" icon for the welcome screen.
Some more OWWidget cleanup.
Added node title background highlights for selected nodes.

(transplanted from orange/572797f576ade81d53a9da0f1bf8fa2c3818ba0d)
Do not truncate an existing file in case of an error in 'save_scheme'.

(transplanted from orange/bdf2527fe91799412a2fa73038e1a6a3a5f783ce)
Fixed an UnboundLocalError in IntesphinxHelpProvider.

(transplanted from orange/53ad9ec6931d495d896f066d0895908e2d030e1c)
Disconnect the SchemeNode from the widget when it has been removed.

(transplanted from orange/dbcc370d05059e2a52d8e3d9291f8fa5031d10c9)
Initialize the SchemeNode state messages on widget initialization.

(transplanted from orange/41d75ac844c92a6df3f9dd311842e386bc3aa1c8)
Delete a reference to a widget when it is deleted.

(transplanted from orange/4d06c658ad17c239a4a86c8cb11ad079501746df)
Fixed NodeItem progress state initialization and updating.

Progress indicator should only be visible when the node is
in a "processing state".

(transplanted from orange/975cc3daadf35924ee5b9ca3af21c4698cdf6b4d)
Fixed an AttributeError in DragStartEventListener.

The 'buttonDownObj' member variable can already be None if another
drag/drop operation starts and completes  within the
'dragStartOperationRequested' signal propagation.

Use the local stack variable to access the button instead.

(transplanted from orange/872337b35d3c9c80650eacc5b2a6147446d17b01)
Ensure the NodeItem shows a visual progress indicator even at 0% progress.

(transplanted from orange/5fd8d0445790928782dda1cfc1a6d3185cc55495)
Make sure the opened widget window is active and has keyboard focus.

(transplanted from orange/13b11307771636e7296c59340243fcf42f27e52d)
Added progress bar state tracking in OWWidget.
Initialize the tracked processing state property.

(transplanted from orange/41a3f530528cee708beaca380dcb26fbf2fb9851)
Fixed widget progress bar initialization.

(transplanted from orange/a34f5c6868697439da3d9b615e56262d9690f2ae)
Adaptive control point positioning to prevent a cusp.

(transplanted from orage/2ec166afa6771d78ff3b62f8ee339ad5516c7a89)