Move main selection index to SelectionList
MultiSelectors now transforms the whole selection
MultiSelectors takes a reference to the current selection and mutate it
Remove simple selectors, always go through a multi selector
Move (keep|flip|remove|clear)_selections from editor method to free selectors
SelectionList should never be empty
Remove Editor::main_selection(|index), directly use the SelectionList method
Rename MultiSelector to Selector
Move template selectors to the header
Remove Editor::select methods, add a non-const selections getter
Fix select_next_match
Remove Editor::rotate_selections
split_lines sets the last selection as the main one
Remove Editor::move_selections
Add Context::selections method, and use it in priority to the Editor's one
Less uses of Editor, in favor of Context
Move SelectMode enum as an implementation detail in normal.cc
Remove insert_hide_sel option
Move insert and erase to normal.cc, and move edition management to context
move InsertMode to normal.hh