Fixes multiple ways to crash QmlDesigner after last .qml file is closed
by enforcing a switch to Edit mode then.

Patch originally done by con.

preferred mode.

Switching to preferred mode is solely done by the editor manager.

QML Designer now listens to contextChanged signal instead of
modeChanged and checks that the context is same as that of qmldesigner.

Reviewed-by: con

While before you had to call add, then remove for each context id and
then call update, now you call updateAdditionalContexts with a list of
contexts to remove and add. It has the update step built in.

Reviewed-by: con

Some Qt Designer actions stayed visible and/or active while editing
source files or when using the QML Designer even though they shouldn't.
Fixed that by applying some manual context handling in the DesignMode.

Reviewed-by: con
Reviewed-by: Lasse Holmstedt <lasse.holmstedt@nokia.com>

... and switch to it in EditorManager::activateEditor.
There no longer is a need for special currentEditorChanged()
handling in the editor that use a special mode.

QmlDesigner receives an additional setting specifying the
desired edit mode. QmlJSEditor detects whether QmlDesigner
is present by checking the registered mimetypes of the
Design mode and retrieves the settings via keys.

Remove some obsolete code and clean up includes
on qmldesignerconstants.h

Reviewed-by: con

Moved Qt Designer to open from Design Mode. Also, Design mode is now global,
and created in coreplugin. Other plugins can register themselves to it.