WelcomePage: Taking text sizes into account for layout

Buttons do not have a fixed width of 160px anymore
and the sidebar layout becomes dynamic.

Task-number: QTCREATORBUG-12108
Task-number: QTCREATORBUG-12091
Change-Id: I940b4b7c9c79a312766501559147abb85833b78f
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>
QmlDesigner.TabViewSupport: Add EnterTabDesignerAction

Change-Id: I381df57461ab3e45e477dc34674e78161342432b
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
QmlDesigner.ContextMenu: Fix regression

We have to create a local SelectionContext for the target node.

Change-Id: I69a490fbc154d5cc15d9ea4f40899afdbab77a7c
Reviewed-by: Marco Bubke <marco.bubke@digia.com>