QML: rework rounded corner windows

 * use a shader based on vertex position, like the one in LSM
 * use an intermediate CardWindow instance between the delegate and the WindowContainer, so that the parent of the WindowContainer is never deleted
 * use simple corner images when transparency is not needed (background image, maximized and fullscreen windows)
 * Remove the DropShadow shader

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>
QML: add two new files to facilitate Desktop development

* the qmlproject file can be opened in QtCreator to work only with qml files
* the LunaNext.js file can be used to simulate the LunaNext C++ module

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>
QML: remove some unused image files, and use the usual LSM image files where appropriate.

More precisely: launcher icons and window corners.

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>
QML: introduce a basic app launcher. It is currently based on a fictive LS2 service provided by a test component.

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>
QML: build up a stub test plugin written in QML/JS to test on desktop.

This solution is better than the previous one as one only needs to open the .qmlproject file to work on the QML structure.
No modification of any QML file is needed.

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>