Use config for selection mark/unmark symbols

[vapourismo]

Configuration items uda.taskwarrior-tui.{un,}mark-selection.indicator are not used when rendering the corresponding symbols in the main table.

Example configuration:

uda.taskwarrior-tui.selection.indicator=a
uda.taskwarrior-tui.mark.indicator=b
uda.taskwarrior-tui.unmark.indicator=c
uda.taskwarrior-tui.mark-selection.indicator=x
uda.taskwarrior-tui.unmark-selection.indicator=y

The result looks like this. x and y are nowhere to be seen.

With this fix it looks like this.