Core: Refactor handling of "window state" actions out of MainWindow

Currently the actions for fullscreen, minimize and zoom only apply to
the main window, even if a different window is active.
Refactor the handling of these actions into a WindowSupport class, and
use that instead for the main window. In a second step, this will be
used to add the functionality to the corresponding external windows
(e.g. help and editor windows)

Change-Id: Ief2c880f40948c3bb724196d6e0cfe888b8ece89
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
EditorManager: Add window actions (fullscreen etc) to editor windows

Change-Id: I478db8c994cc3126d8d518e3d574e9adfa600ad1
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Help: Add window actions (fullscreen etc) to external help window

Change-Id: I1486bdac965e92990013e77c9ea4d26727dff0f9
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
LoadCoreDialog: widget focus order fixes

- On Tab the widget focus now jumps left-to-right--top-to-bottom

Change-Id: I3ebf56f4f4b3325ed60c9667bbdbbe6f77d3f9cb
Reviewed-by: hjk <hjk121@nokiamail.com>