GUI additions, behavior changes, and code cleanup

Added column to TrackTable that has a checkbox to say whether or not
you want to convert that file. Uncheck files you don't want converted.
Track # column text is now centered instead of right aligned.
Cell grid now displays properly in TrackTable.

Stopped AlbumArtPanel from automatically reloading/erasing iTunes album
art when the artist or album tag has changed. It will only reload album
art when the artist and/or album has changed and the next or previous
button is pressed.

Renamed what used to be ConvertModel to a class called Settings. It
handles loading all settings from Settings.properties at the start of
the program.
Created new ConvertModel class.
Moved a bunch of variables and methods from ConvertButtonController
into ConvertModel.

Added some public and private helper methods to TagsModel.

Disabled JAudioTagger log statements.

Note: New settings have been added to Settings.properties but they
don't do anything yet.