gdk3-no-gi: move ext/ dir ext/gdk3/ -> ext/gdk3-no-gi
gdk3-no-gi test: rename `gdk3` -> `gdk3-no-gi` and rename it's base path
gtk3: use gtk3-no-gi to test in Travis CI.

This is workaround. Currently, gtk3 depends gdk3-no-gi.
gtksourceview3: use gdk3-no-gi to test in travis CI.

This is workaround. Currently, gtksourceview3 depends in gdk3-no-gi.
vte3: use gdk3-no-gi to test in Travis CI.

This is workaround. Currently, vte3 depends gdk3-no-gi.
gtk3 test: use gdk3-no-gi for tests.

This is workaround. Currently, tests for gtk3 depends gdk3-no-gi.
gdk3 test: Gdk::EventCorssing.{x,y} returns kind of `Float`
gdk3 test: gi based Gdk::Event* has `#type` instead of `#event_type`
gdk3 test: rename constants kinds_of assertions