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Avoid a crash when browsing hidden directories

If viewing hidden directories is disabled, EasyTAG will fail to find a
hidden path in the directory model, and will crash when trying to
retrieve the full path from the model. Avoid this by returning early
from Browser_Tree_Select_Dir().

https://bugzilla.gnome.org/show_bug.cgi?id=730326
Move main window to EtApplicationWindow object
Pass the main window to actions as user data
Move log area to EtLogArea object
Move playlist window to EtPlaylistDialog object
Move load filenames dialog to EtLoadFilesDialog
Move file search dialog to EtSearchDialog object
Move preferences dialog to EtPreferencesDialog
Move CDDB dialog to EtCDDBDialog object
Move scan dialog to EtScanDialog object
Replace strncat() with g_string_append()
Move select and unselect actions to main window
Move file selection actions to EtApplicationWindow
Set a rough initial window size and pane positions
Move browser to EtBrowser object
Avoid a GtkApplication assertion

Realizaing an application window during construction triggers an
assertion with older versions of GTK+, and a warning with newer ones, so
only show the window after it has been associated with the application.
Add GSettings schema

Leave the keys that depend on enums and flags commented for now, as the
enum values need to be generated with glib-mkenums.
Use GSettings for most application settings