Abstract libwacom database initialization into a single place

No real changes for the non-tablet code, but for tablets we now keep the
libwacom datbase around. The primary motivating factor here is response time
during tests - initializing the database under valgrind took longer than the
proximity timeouts and caused random test case failures when a proximity out
was triggered before we even got to process the first event.

This is unfortunately a burden on the runtime now since we keep libwacom
around whenever a tablet is connected. Not much of an impact though, I
suspect, chances are you're running a web browser and everything pales against
that anyway.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>