[WPF] Widget allocation optimization

Box: Only measure the child widget when it is first added to the box.
[WPF] Better defaults for stepping increments in ScrollView
[GTK] Fix CloseRequested event for Dialogs
Explicitly assigned readonly fields and fixed a weird conditional check.
Allow closing dialog that's run more than once
Merge pull request #254 from Therzok/master

Fix issues with unassigned readonly fields and a weird conditional check.
Fix allocation issue in FrameBox
CloseRequested event fixes

Changed the meaning of the return value of OnCloseRequested. If it returns
true, the window will be closed. The event args now has an AllowClose
proeprty. Implemented for the mac backend.
Added some null ref checks

Throw proper exceptions if trying to use drawing api before
initializing XWT.
Allow closing windows by default
[GTK] Fix crash

Caused by collection change while being iterated.
[GTK] Fix dialog closing issue

Close a dialog when calling Respond in the CloseRequested handler.
[Xwt.Wpf] Don't call CloseRequested machinery when a dialog loop is being ended.
[Mac] Fix vector image rendering issue

Flip the image if the target surface is not flipped.
Use all StockIcons properties instead of fields.
[WPF] Implement WpfDesktopBackend.GetScaleFactor
[GTK] Fixed label alignment when in wrapping mode

Also added unit tests for label alignment
[GTK] Fix crash when trying to generate a bitmap that's too small
[GTK] Another fix for label alignment
Fix anchor parsing issue in FormattedText

When parsing an anchor element, allow relative uris.
Added unit tests for FormattedText and LinkLabel.