EShellView now holds a reference to the active GalViewInstance. Where
applicable, the EShellView subclass is responsible for keeping this up
to date when the sidebar selection changes.
Holding a reference allows EShellView to implement common actions like
"Save Current View" directly instead pushing it on to subclasses.
New functions:
e_shell_view_get_view_instance
e_shell_view_set_view_instance
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