main window statusbar
QStatusBar gives almost no control over display and placement
of child widgets. It's not possible to subclass and reimplement
either, due to how QMainWindow works internally, and also due to
the special handling for the size grip and other platform specific
handling in QStatusBar.
Instead, we embed a single QgsStatusBar covering the whole real
status bar. All child widgets and temporary messages instead
are pushed to the QgsStatusBar instead - giving us as much control
as we desire over how these widgets are placed and their behavior.
As a result the locator widget has been moved to its logical placement
on the left of the status bar.
All plugins must ensure that they use the status bar interface
available via iface.statusBarIface() instead of directly interacting
with the status bar (e.g. iface.mainWindow().statusBar()...)
