gui: set keyboard focus when switching between dock widgets (bug #36957) * main-window.cc (handle_command_window_visible, handle_command_history_visible, handle_current_directory_visible, handle_workspace_visible, handle_editor_visible, handle_documentation_visible): slots for signal visibilityChanged, emitted when widgets get visible * main-window.cc (construct): connect signal visibilityChanged to slots * main-window.cc (focus_editor): call editor's own function for setting focus * main-window.h: declaration of new slots * file-editor.cc (set_focus): new function: setting focus to actual editor tab * file-editor.cc (add_file_editor_tab): connect signal fetab_set_focus to the slot set_focus of file_editor_tab * file-editor.h: new function set_focus and new signal fetab_set_focus * file-editor-interface.h: new virtual function set_focus * file-editor-tab.cc (set_focus): new slot for singal fetab_set_focus from file_editor, setting the focus to edit area * file-edtortab.h: new slot set_focus

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