[NUI.Gadget] Add NUIGadget class for inhouse developers (#4935)

[NUI.Gadget] Add NUIGadget class for inhouse developers (#4935)

* Implement NUIGadget

Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>
* Adjust project file

Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>
* Update csproj

Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>
* Update implementation

Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>
* Remove dll unload

Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>
* Remove unnecessary file

Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>
* Update sln file

Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>
* Add missing dependencies

Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>
* Add a missing property

The type is added.

Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>
* Update implementation

Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>
* Fix typo

Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>
* Check return type of Assembly.CreateInstance()

This patch uses 'as' keyword to check the instance type.
If it's not NUIGadget, the return value will be null.
In that case, the Add() method throws an exception.

Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>
* Add abstract keyword to NUIGadget class

Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>
* Modify Create() method of NUIGadget

If the Create() method returns 'false', the Add() method of NUIGadgetManager
throws an exception.

Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>
* Use CoreApplication.Post() instead of TizenUISynchronzationContext.Post()

Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>
Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>