Added Clipper Library and DocumentSelection class.

*Not fully implemented yet!*
Completed Magic Wand selection modes.

Also added some code to the DocumentSelection class and made the new
"double selection storage" system (consisting of a Cairo.Path and a
List<List<IntPoint>> (no longer using the Clipper to store Polygons))
work all over Pinta (hence the many files that were slightly modified).
Added complete Ellipse Polygon calculations.

Also added more comments in the DocumentSelection class and modified
some History Items a little.
Fixed DocumentSelection.Clone method.

Before, the DocumentSelection.Clone was using "shallow" cloning instead
of "deep" cloning. This was fixed by making the Clone method manually
make a copy of each variable in the DocumentSelection class.
Fixed LassoSelectTool.

When the lassoPolygon was being cleared after the mouse was released
(and after it was added to the doc.Selection.SelectionPolygons
collection), the Polygon inside SelectionPolygons was also being cleared
since it was only referencing to lassoPolygon rather than being a copy.
I fixed this by including System.Linq and then calling the ToList method
on lassoPolygon to make a complete copy of the Polygon.
Added ToolBar for selection modes for the MWT.
Changed MWT selection modes DropDown to ComboBox.

Also updated translations file with new Magic Wand Tool text.
Fixed line width and translation problems.
Optimized calculation of elliptical curves.

The CalculateCurvePoints method was reduced to about 46.46% of the time
(ticks) it took for the old calculation code to execute, on average.
This can be multiplied by 4 for each curve of an ellipse and then by the
number of ellipses that are calculated when the mouse is being dragged.
Overall, it makes the elliptical calculations much faster.
Undid some code that doesn't work in Windows.

It only works in Linux, and I'm using Windows, so with Cameron's
approval I'm teporarily undoing this code until the problem is fixed (so
that I can continue working).
Added cursors and paintbrush thickness cursor.

Also fixed a null error that occurs in BaseBrushTool when the value of
BrushWidth is retrieved and the ComboBox hasn't been set up yet.
Improved on paintbrush thickness cursor icon.
Implemented thickness icons on 4 tools.

Also added 2 methods to the BaseTool class for creating the icons used
for these 4 tools.
Fixed zoom problem with thickness icons.
Improved JPG export compression quality dialog.

Now, the JPG export compression quality dialog:
- only displays when the user clicks Save As, not Save.
- remembers the previously exported JPG compression quality, even when
Pinta restarts.
- has the default control being the OK button.
- shouldn't disappear anymore (on some OS's, like Windows XP) when Pinta
is minimized/unfocused and then restored.
Modified JPG compression quality setting usage.

Also improved comments and variables.
Fixed Document switching cursor changing problem.

Also removed 2 sentences of comments that were unnecessary.
Removed some unnecessary private variables.
Added/improved some cursors.
Removed custom mouse cursors from 3 tools.

The custom mouse cursors for the Gradient, Ellipse Select, and Rectangle
Select tools were removed. I had added some custom mouse cursor code for
5 tools earlier (including these 3), but it was decided that it would be
best to leave them with the standard mouse cursor for now.