Clean up in use of DesktopRootWindowHostXWalk

DesktopRootWindowHostXWalk serves as our "desktop environment"
per scree, which is somewhat similar to the X11 window manager
concept.

The specialized version of DesktopRootWindowHost (*XWalk) in
our situation, is created as part of DesktopNativeWidgetAura
initialization, but requires build system changes for doing so.

This patch cleans up part of the code, making it very clear
why we manually creates DesktopRootWindowHostXWalk and why it
works. It also removes the ::Create method which is called
by DesktopRootWindowHost because it is wrongly named, and if
named correctly it will conflict with the symbol from *X11.
Merge pull request #701 from kenchris/cleanupaura

Clean up in use of DesktopRootWindowHostXWalk
[Tizen] Add pkgconfig(scim) to the list of crosswalk dependencies.
Merge pull request #710 from alexshalamov/add_scim_dependency

[Tizen] Add pkgconfig(scim) to the list of crosswalk dependencies.
Update the documentation about Sync Messages

Now it is made clear that replies to Sync Messages can be sent after
the handler of the call has returned.

This is valid because we can only have one Sync Message going on at
any time (the renderer is blocking).
Merge pull request #712 from vcgomes/execution-model-docs

Update the documentation about Sync Messages
Clean the house in XWalkExtension(Instance)

- Updated the comments to be more on par with existing implementation;
- Added an overview at the top;
- Removed unused includes, forward declarations;
- Moved Callback types to XWalkExtensionInstance;
- Shuffled some functions to their appropriate places.

This patch shouldn't change code behavior.
Rename XWalkExternalContext to XWalkExternalInstance

The concept is now called Instance instead of Context. This allow us to
remove the typedef hack in XWalkExternalAdapter.
Rename xwalk_external_context.cc/h to xwalk_external_instance.cc/h

To match the class names. Patches are separated so the name change in
code is easier to review. This patch just renames the files and updates
references to it.