Plugins: adds PluginMenuInserter helper (#7419)
Plugins: adds PluginMenuInserter helper (#7419)

Allows user-plugins to register themselves
into "More tools" menu.
Frenzie/koreader,mwoz123/koreader,NiLuJe/koreader,Frenzie/koreader,NiLuJe/koreader,poire-z/koreader,koreader/koreader,poire-z/koreader,koreader/koreader