Unify how extra selection are added to the editor

[rlaverde]

The idea is to move how extra selections for cell, lines, search and others that use these functions:
https://github.com/spyder-ide/spyder/blob/master/spyder/widgets/sourcecode/base.py#L319

Move them to use TextDecoration API:
https://github.com/spyder-ide/spyder/blob/master/spyder/widgets/sourcecode/api/decoration.py

This is needed for #4289

[rlaverde]

This is also causing a little error: Selecting a codefolding arrow will add codefolding extraselection, but editor extraselections will be removed (current line, current cell, highlight words)