Fix mac universal build

For weird reasons, having -Werror=missing-prototypes break zeromq build in universal mode (it links in 64 bits while building in 32 bits)
Extend ShowNotification utility method

Will be used by ServiceAPI
Add support for "error" type notification and add visibility field

Uses new extended ShowNotification method
Add error and visibility option to mythutil
Add SendNotification to Myth and Frontend service API
Fix trailing whitspaces