Allow assigning hotkey to launch user-defined scripts

This commit makes it possible to create hotkey to launch user-defined bash scripts from ~/.config/dolphin/scripts. Load the scripts as actions and deletes old scripts from dolphinui.rc when creating dolphinmainwindow. The scripts are launched from current URL as working directory and they get selected items as arguments.

Example script to test:

echo -e "script:\n$0"
echo -e "working directory:\n$(pwd)"
echo "arguments:"
for arg; do echo "$arg"; done

Creates a workaround for BUG: 260266 and most probably doesn't have the security risks of !206 (closed)