Fixed a few crashes because of dangling pointers

The two list stores from the artist/album view contain pointers to the
list in ETCore->ETArtistAlbumFileList. When the complete list was freed,
the GUI still used the pointers in the list stores and crashed.

I saw segmentation faults while toggling between artist/album and
directory view and furthermore while shutting down EasyTAG.
Fixed a album/artist view memory leak

In artist/album view the album list store contains a row for a
collection of all albums for an artist - the "<All Albums>" row. The
attached GList was not freed.

Add a column with a boolean value to the model to be able to identify
this row, as a simple string comparison seems unsafe.
Avoid assertion in Action_Select_Browser_Style()

When changing to artist/album view the browser list gets cleared.  While
clearing, GTK+ emits a changed signal for each row, and
Browser_List_Row_Selected() gets triggered, unfortunately with a row
where the backing file list has already been freed.

Added a function which clears the list and stops emissions of
Browser_List_Row_Selected() while clearing.