`gtk_builder_get_parameters()` is a hot path, being called twice for
each object in each UI file in an application. The majority of objects
have ≤ 8 properties, which are each filtered into either `parameters` or
`filtered_parameters`.

Unfortunately, both of those arrays are created as empty `GArray`s, and
adding 8 elements to an empty `GArray` hits the worst possible case of
reallocating and `memcpy()`ing the array 3 times. As the array size is
doubled with each reallocation, the cost is not particularly well
amortised when the array size is small.

From the `ObjectInfo`, we actually know how many properties there are in
total, so just allocate the arrays at the right size to begin with.

This saves 7% of the instruction cycles needed to start up
gnome-software to the point where it’s showing its main window,
according to callgrind. gnome-software is making around 5500 calls to
`gtk_builder_get_parameters()`.

Signed-off-by: Philip Withnall <withnall@endlessm.com>