Insert special character tool

A tool to add special characters and diacritics to text.

Also added a new button type ve.ui.GroupButtonWidget that includes a
group of PushButtonWidget objects and returna the individual button's
value upon click

Wikis can edit <visualeditor-specialcharinspector-characterlist-insert>,
a JSON string, to include their own desird special characters to insert
through the tool.

Bug: 50296
Change-Id: I26d1f437feef1c8b61ed3be5f74ef524b33baf49