markword: A new "highlight all occurrences of selected text" mode.

It's more comfortable for me than the kbd shortcut or double click.
It has a side effect, a "find with highlight" functionality when
using search-field on the toolbar, because current match will be
selected by the find.

KATE, Kwrite, NetBeans, etc also offers such functionality.

Disabled by default, enable it in addons plugin's preferences.