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 ()

--- a/libgui/src/settings-dialog.h
+++ b/libgui/src/settings-dialog.h
@@ -24,6 +24,9 @@
 #define SETTINGSDIALOG_H
 
 #include <QDialog>
+#ifdef HAVE_QSCINTILLA
+#include "lexer-octave-gui.h"
+#endif
 
 namespace Ui
 {
@@ -39,6 +42,10 @@
 
 private:
   Ui::settings_dialog * ui;
+#ifdef HAVE_QSCINTILLA
+  void read_lexer_settings (QsciLexer *lexer, QSettings *settings);
+  void write_lexer_settings (QsciLexer *lexer, QSettings *settings);
+#endif
 };
 
 #endif // SETTINGSDIALOG_H