lots of new allowed extensions, small fixes

# Please enter the commit message for your changes. Lines starting
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# On branch alpha
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
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#	modified:   tomamp/mainwindow.cpp
#	modified:   tomamp/playlistmanager.cpp
#	modified:   tomamp/playlistmanager.h
#
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
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#	oold/
#	tomamp-build-desktop/
#	tomamp-build-simulator/
#	tomamp/images/.directory
#	tomamp/tomamp.pro.user
