[Mac] Fix CanvasBackend MouseEntered, MouseExited, MouseMoved events

Add a TrackingArea to CustomWidgetBackend to capture events.  Also fixes these events for Boxes and other users of CustomWidget
[Icons] New searchbox icons:
* searchbox-clear
* searchbox-search
[WPF] Fix false scroll orientation
Merge pull request #302 from jfreax/WPFScrollbarOrientation

[WPF] Fix false scroll orientation
[Gtk] Correctly convert the Command key modifier
Add three more cursor types

Move, wait and help cursor.
Gtk and WPF only. There are no such standard icons on MacOSX. Mac user will see the standard cursor.
Merge pull request #305 from jfreax/more-cursortypes

Add three more cursor types
Update CanvasWithWidget sample to show Mouse Enter, Exit and Move events
from upstream
Allow Initialize without specifying the backend type

Basically commit 6ce8538719d541973a87fd6d77e6ddacf55771b5 made it
impossible to use Xwt dynamically. This commit will revert to a previous
(working!)  state by removing the null check.
[Icons] Fix missing icon for search entry.
add XwtComponent.Tag property
#300: fixed tabs and replaced oldschool property with auto-implementing variant of one
[Mac] Fix events for Label, LinkLabel, TextEntry Backends,
and update WidgetEvents sample to demonstrate better

WidgetView has been moved into a separate file WidgetView.cs, to
handle and dispatch the events generated by NSView and its TrackingArea.
This is used as the base for CustomAlignedContainer for Label, etc.
[WPF] Defer calling Widget.Focus() until it is loaded

This fixes a weird issue where a TextBox would not appear at all
if Focus was called too early.
WPFBackend.KeyboardUtil: use refactoring-proof consts to calculate Key
Fix issue #289: unable to use StockIcons in Gtk ListView
Flush project changes
[GTK] Fix checkbox cell view issue

Fixes issue #293: cannot check CheckBox in ListView
Merge pull request #311 from atauenis/master

Add XwtComponent.Tag property