scide: improve default paths in open/save dialogs
scide: save document dialog: always allow saving with any extension

[Fixes #615]

* Do not try to pre-select one of the file name filters - it doesn't
  necessarily work with native dialogs.

* Put "All Files" filter on top, so it's always first selected; other
  filters may automatically change suffix in native dialogs, even when
  file name is pre-selected.

* Do not use QFileDialog::setDefaultSuffix(), because it does not display
  a warning when file path with auto-added suffix already exists.

* Improve file name filter labels