add selecticonstring and contextmenu support

[Jaksuhn]

resolves (partially) #6 (I don't remember where SelectionString comes up)

requires https://github.com/NightmareXIV/ECommons/pull/64

selectstring and selecticonstring, despite being basically identical, aren't interchangeable via addonmaster so there's a lot of duplicated code there. This was kind of a lazy expansion

Context menu support only works for native entries (not dalamud added ones since those do not have callback values)

[Limiana]

thank you!