[statusbar-] use '' for BaseSheet selectedStatus

[midichef]

Closes #2599.

'{selectedStatus}' appears in the GraphSheet statusbar as a raw string instead of a value. That's because selectedStatus can't be evaluated for BaseSheet.

[saulpw]

Once upon a time the graph sheet reflected the selection on the source sheet (see the unused color_graph_selected and other remnants). I believe it's still possible to select/unselect elements this way, but the selection itself is invisible. (I'd love to get that working again, but that's outside the scope of this PR.)

[saulpw]

I edited the PR to add a separate BaseSheet.property for selectedStatus that returns an empty string. OOP overloading instead of duck-typing. Note that this is untested code (though I don't see how it wouldn't work..)

[anjakefala]

Upon reproducing, this came up:

Traceback (most recent call last):
  File "/home/anja/git/visidata/visidata/sidebar.py", line 115, in drawSidebar
    sidebar, title = sheet.help_sidebars[vd.disp_help%len(sheet.help_sidebars)]()
                                         ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
ZeroDivisionError: integer modulo by zero

[anjakefala]

Ah, this is unrelated! I will open an issue and investigate this separately. The core issue has been resolved.