Move utils_str_remove_chars() from the plugins/splitwindow.c to src/utils.c and add it to the plugin API.
Make utils_str_remove_chars() work on a new copy of the input string instead of modifying it in place.

git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@4003 ea778897-0a13-0410-b9d1-a72fbfd435f5