Create UICommons project to contain shared addins UI

Move AboutXWord, AddinSettingsForms, AddPageForm and XWiki Tree component to a new assembly, UICommons, that can be used on XWord2003 and other addins.
These UI elements should be VSTO- and XWikiAddin-independent.
If necesary, the forms will have public events for their actions (ex: OnApply,OnOK, OnCancel), and the XWikiAddin will register for these events.