Fix string comparison in object type filter

[bshankstd]

The object type filter used strncmp() to compare object
type names, but limited the comparison to the length of the
filter string. For example, a filter of 'GstPad' would display
both 'GstPad' and 'GstPadTemplate' types.

Use g_strcmp0() instead, it uses the entirety of both strings
when comparing.

[danni]

Hi @bshankstd, that was intentional to be able to match all objects of a given group. There's a TODO written somewhere to replace the filter with a globbing function or regular expression, that I never got around to implementing.