[HatoholArmPluginInterface] Add a new methods: set/getGLibMainContext().

Currently only one GLib Main context is used for timer, GIOChannel
(mainly for pipe) except for libsoup's handling. However, when the
number of the GLib's event loop's user increase, this architecture
doesn't take care of that.
Merge pull request #694 from project-hatohol/hapi-set-get-glib-main-context

[HatoholArmPluginInterface] Add a new methods: set/getGLibMainContext().