add bookmark functionality to the documentation browser (bug #54938) * libgui/src/documentation-bookmarks.cc: new file for the bookmarks tab; (documentation_bookmarks): initialize the tab, the filter and the tree; prepare the bookmark file and read existing bookmarks; (~documentation_bookmarks): empty destructor; (add_bookmark): slot for the adding bookmark action, get current title and url; (add_bookmark): do the adding, possibly as child of a parent item; (add_folder): slot for context menu action for adding a folder, check current position where to insert the new folder; (add_folder): do the folder adding; (filter_bookmarks): filter bookmarks following the changed filter text; (filter_activate): enable/disable filter; (update_filter_history): save a search term when acknowledged by return; (handle_double_click): open the clicked bookmark; (ctx_menu): show context menu at current mouse position; (open): open a bookmark via context menu; (edit): edit a bookmark via context menu; (remove): remove selected bookmarks via context menu; (show_filter): toggle visibility of the filter; (save_settings): save settings and initiate writing the bookmarks; (write_bookmarks): open file and initiate writing all top level items; (write_tree_item): write a single item and its children; (read_bookmarks): open the file, check if it is valid and loop over all top level items for reading them from the file; (read_next_item): read an item and its children in case of a folder; * documentation-bookmarks.h: class documentation_bookmarks derived from QWidget, declaration of all required functions and class variables; * documentation-dock-widget.cc (save_settings): new derived method, emitting a signal for saving setting in child widgets and callinf the original method * documentation-dock-widget.h: derived method save_settings and signal for child widgets * documentation.cc: include documentation-bookmarks.h; (documentation): add bookmark tab, connect signal for saving settings; (add_action): check reveiver before connection the actions signal; (construct_tool_bar): add toolbar button for adding a bookmark, connect its signal the add_bookmark slot and connect signal for saving settings; (notice_settings): add shortcut for adding a bookmark; (update_history): move combining page title and current anchor into a more specific title ... (title_and_anchor): to here; * documentation.h: new function title_and_anchor, new action * gui-preferences-dc.h: new file with constants for some bookmark settings and for the xbel file syntax * gui-preferences-sc.h: default value for bookmark action * bookmark-new.png/bookmark-new.svg: icon for tool bar button * icons_license: add new icon * module.mk: add new files * shortcut-manager.cc (init_data): initialize new short for bookmarking