Hover-sensitive pointer for Nitpicker with VirtualBox shape support

Per default the standard "big mouse" pointer is rendered on screen,
which is the behavior known from the classical app/pointer.
Additionally, VirtualBox pointer supports to render "pointer shapes"
when hovering configured Nitpicker sessions. The policies can be
defined for labels or domains of the sessions.

! <start name="vbox_pointer">
!   <resource name="RAM" quantum="1M"/>
!   <config>
!     <policy domain="smiley"          rom="smiley"/>
!     <policy label="test-label-arrow" rom="arrow"/>
!     <policy label="test-label-blade" rom="blade"/>
!   </config>
! </start>

In the example above, which is from vbox_pointer.run, the domain
"smiley" gets the ROM "smiley" as pointer shape. The labels
"test-label-blade" and "test-label-arrow" will render the ROMs "arrow"
resp. "blade" as pointer shape. Note that label matching is done from
the start of the actual label until the defined label ends. So,
"test-label-blade2" will also match the policy defined above.

The most common use case for vbox_pointer is VirtualBox, which reports
the guest-pointer shapes if Guest Additions are installed.