#414 Fix GTK+ 3 build when the GTK+ version is < 3.20

[Alberto Garcia]

One user reported that the GTK+3 build is broken if GTK+ is < 3.20

This is because commit 9e17a23633b4 introduced the GdkSeat API (which requires GTK+ >= 3.20) without updating the GTK_CHECK_VERSION() lines to reflect it.

I verified that updating these checks is enough, the deprecated code builds with older versions of GTK+3 and also with GTK+2.

[Sergio Baldoví]

Thanks. Committed in [28b8db].