GVariant: add support for single precision floats

GVariant: add support for single precision floats

Add a new type 'f' to correspond to single precision floating point
values.

This type was never added to D-Bus for two reasons:

 1) there is no benefit to using float rather than doubles as parameters
    for RPC

 2) classically, you shouldn't move bulk data over D-Bus

Now that we've decided that we want to use D-Bus for bulk data
transfers, it makes a good deal of sense to want to send an array of
floats or an array of fixed-sized tuples containing floats.

https://bugzilla.gnome.org/show_bug.cgi?id=740897