Fix problem with removing t:ctrlspace_start_window

Now there is ctrlspace#bufnr() function which returns the
plugin buffer number if it exists or 0 otherwise.
In that way one could check if the plugin is visible.
It can be useful e.g. in a status line modifier:

    function! CtrlSpaceStartWindowIndicator()
      return ctrlspace#bufnr() && t:ctrlspace_start_window == winnr() ? g:ctrlspace_symbols.ia : ""
    endfunction