Added an interface settings tab to the settings dialog. Made a few improvements on the editor. Wrote a small welcome text in the welcome wizard. Moved configuration file to ~/.config/octave-gui/settings. * FileEditorMdiSubWindow: Set sources for code completion to all, lowered completion threshold, changed colors. * SettingsDialog: Added interface tab with shortcut configuration table widget. * WelcomeWizard: Wrote a little text to appear at startup.