Add a command line option and a Mousepad setting to choose how to open files

Closes #14 (closed).

I made the following design choice: only one --opening-mode option, with three modes:

• "tab": open files in new tabs in the last active window if any, or in a new window;
• "window": open files in new windows;
• "mixed": open files in new tabs in a new window (this is the current behavior).

I thought this would be better than several options, like --window and --tab, which can lead, in my opinion, to behavior that is not as intuitive as the option names might suggest, as e.g. for Gedit or xfce4-terminal.

When adding the "opening mode" entry to the preferences dialog (on which the command line option have precedence), I also modified the expansion properties of some combo boxes.