gui: individually configurable styles of the editor lexers * color-picker.cc/.h: new class for a simple color picker * libgui/src/module.mk: added color-picker.cc/.h * file-editor-tab.cc(update_lexer): read config of the lexer from settings file * file-editor-tab(notice_settings): editor font not directly configured from the settings file but via the lexer settings in update_lexer * lexer-octave-gui.cc(constructor): inherits from QsciLexerOctave instaead of QsciLexer, code cleanup * lexer-octave-gui.cc(language,lexer,defaultFont,defaultColor,description): removed, original methods used and configured via settings * lexer-octave-gui.h: removed functions language,lexer,defaultFont,defaultColor, description and enum; using methods of octave lexer instead * settings-dialog.cc(constructor): editor font not configured from settings, defining lexers for supported languages and reading their actual settings from the settings file (via read_lexer_settings) * settings-dialog.cc(read_lexer_settings): new function for reading the lexers settings from settings file and creating the tabs with interactive elements for configuring these settings * settings-dialog.cc(write_changed_settings): remove settings for editor font, defining lexers for supported languages and writing the settings from the dialog into the settings file (via write_lexer_settings) * settings-dialog.cc(write_lexer_settings): new funciton for getting the new lexer settings from the interactive elements and write them into the settings file * settings-dialog.h: new functiond read_lexer_settings, write_lexer_settings * settings-dialog.ui: new tab for the editor styles which contents is dynamically created in settings_dialog ()