changed checking for NULL a bit

the code below clearly assumes that slide should never be NULL,
so the check is now done via g_return_if_fail
Merge pull request #27 from monsta/nulls-must-die

changed checking for NULL a bit
Added keybingings migrate script to postinst
keybinding migration script: add run-once check
Remove postinst script for keybindings.
2.4.1