* Reworked ExportMainDialog in preparation for the creation of CD and region export dialogs:
* Separated ExportPresetSelector and ExportFileNotebook from ExportMainDialog
* Made ExportTimespanSelector polymorphic regarding single/multiple timespan mode
* renamed ExportMainDialog to ExportDialog and made it easily customizable
* created ExportRangeDialog and ExportSelectionDialog, these can be later customized more if necessary
git-svn-id: svn://localhost/ardour2/branches/3.0@3834 d708f5d6-7413-0410-9779-e7cbd77b26cf
