Doc: fix terminology in the build and run tutorial

"Welcome page" should be "Welcome mode".
Update screen shots.

Change-Id: Iaacf7f014d18ce73a3444ee820e342a9813cf939
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Doc: update screenshot of Qt Quick editors

Change-Id: I46b802c9c2eb72fbc7d14f95d5f845bfda17c9c7
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>