Don't specify "Image" in title of importer since it supportis sif too.
Use Import instead of Import Image, since we can import other formats too.
Add file filters into import dialog
Use rename filter Valid files to Supported files according to the input by Dave Jeffery
Better wording and add mime for audio and images
Add file filters for open file dialog.
Reposition open history and cancel buttons
Enable Open History Button by default.

The implementation plan is make it file extenstion sensitive,
enable only when *.sfg file selected.
Gtk::FileChooser::signal_selection_changed
etl::filename_extension(filename) == ".sfg"
Add a file type selector to save file dialog.

Add a file type selector to save file dialog, so that there is not need
anymore to keyin file extension manually while save/save as document.
Typo and remove unwork code.
Use files instead of documents for file filters of import dialog
Merge branch 'jcome/dialogs'

* dialogs:
  Use files instead of documents for file filters of import dialog
  Typo and remove unwork code.
  Add a file type selector to save file dialog.
  Enable Open History Button by default.
  Reposition open history and cancel buttons
  Add file filters for open file dialog.
  Better wording and add mime for audio and images
  Use rename filter Valid files to Supported files according to the input by Dave Jeffery
  Add file filters into import dialog
  Use Import instead of Import Image, since we can import other formats too.
  Don't specify "Image" in title of importer since it supportis sif too.