[keymap] new ACTION_MENU key to focus sidebar menu item

This adds a new key ACTION_MENU which is translated to "menu" to use in keymaps. The key is used to focus the first button control and in pvr the corresponding button control of the current view within our sidebar menu. 

The initial intention was to quick access the sidebar menu if you stay in epg grid, because top/buttom/left/right is used for scrolling and you can't access the menu quickly. As @da-anda said, it would be make more sense if we could get this in all related windows, so here it is.