possibility to hide filters in history and workspace view (bug#45428) * history_dock_widget.cc(construct): use settings file for show/hide the filter; (~history_dock_widget): save current filter state to settings file; (ctxMenu): add menu entry for showing/hiding the filter; (handle_contextmenu_filter): slot for the new menu entry * history-dock-widget.h: new slot for new menu entry, new class variables for filter and show/hide-state * workspace-view.cc(workspace_view): use settings file for show/hide the filter; (~workspace_view): save current filter state to settings file; (contextmenu_requested): add menu entry for showing/hiding the filter; (handle_contextmenu_filter): slot for the new menu entry * workspace-view.h: new slot for new menu entry, new class variables for filter and show/hide-state

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