Detect keyboard shortcuts with only single modifier keys on key release

This commit allows xfce_shortcuts_grabber_event_filter to detect KeyRelease events from modifier keys. This allows one to have Super_L set to open a menu while Super_L+T opens a terminal.

This is work in progress because:

• I can't get this to work with XFWM; if a menu is to open with Super_L, using Super_L+Left does not switch to the left workspace
• I would like suggestions on what (other) modifier keys to include
• I'm not 100% sure about the formatting

Fixes #1 (closed)