[#433] GuiHandle#setTextField(String, String) should call Gui methods on JavaFx threads (#434)
[#433] GuiHandle#setTextField(String, String) should call Gui methods on JavaFx threads (#434)

On some computers, GuiHandle#setTextField() throws an
IllegalStateException, indicating that the methods called in
GuiHandle#setTextField() should have been executed on the JavaFx Gui
thread.

Let's fix GuiHandle#setTextField() to call Gui modification methods
on the JavaFx Gui thread.
CS2103R-Eugene-Peh/addressbook-level4,se-edu/addressbook-level3,damithc/addressbook-level4,CS2103R-Eugene-Peh/addressbook-level4,damithc/addressbook-level4,damithc/addressbook-level4,CS2103R-Eugene-Peh/addressbook-level4