gdk3: remove "get_" from specific method name

Because it's for conciseness and backward compatibility.

For example:

    Gdk::Pixbuf.get_from_window ->
    Gdk::Pixbuf.    from_window
                ----
gdk3: simplify
gdk3 test: suppress a warning

    Gdk-CRITICAL **:gdk_pixbuf_get_from_surface: assertion 'width > 0 && height > 0' failed
gdk3 test: check whether valid object instead of not raising an exception
gdk3 test: check whether valid object instead of just checking class
cairo-gobject: suppress a warning

    warning: instance variable gboxed not initialized
gdk3: use to_pixbuf for converting to pixbuf

    Gdk::Pixbuf.from_XXX ->
    XXX.to_pixbuf
gdk3: don't use "Gdk::" in loader

Because "Gdk::" is not constant value in the loader. It is the
variable named @base_module in the loader.
gdk3: add "?" to Event#send_event

Because it is a predicate.
gdk3: add "?" to EventFocus#in

Because it is a predicate.
gi: add "?" to boolean field value reader
gdk3 test: add "?" for predicate field reader
gi: support mapping function info to method
gdk3: support Gdk::EventMotion#request
gdk3: use define_method for to_pixbuf methods
gdk3: use define_method for Gdk::Rectangle methods
gi: don't handle MethodInfo as FunctionInfo
gtksourceview3 test: add load path
gtksourceview3 test: use #to_s to assertion
gtksourceview3 test: add property test