[Forms] Add dialog button spec

Add spec for a button which should show a dialog for
input on a single value, to support Image properties
in Fixed Position view, WTD-881.

platform/forms/res/templates/controls/dialog.html

@@ -0,0 +1,4 @@
+<mct-control ng-controller="DialogButtonController as dialog"
+             key="'button'"
+             structure="dialog.getStructure()">
+</mct-control>