shiny-ui-base, state, app: Export MS2 spectra.

* state.R (encode_ms2_to_line, pack_ms2_w_summ): Functions to pack ms2
data based on selected entries in `summ'.

* app (ms2_spectra_tab_name, ms2_spectra_tab_b): New widgets.

* shiny-ui-base: Added input$ms2_spectra_tab_b observer.