Use input context in player::sort_armor

Basically the same as the other PRs.

Something to consider:

The standard help dialog (action `HELP_KEYBINDINGS`) displays the keybindings and allows to change them. That action is bound to the '?' key and is nearly always enabled when the `input_context` is used. It should be extended to display arbitrary text (at the bottom?) that acts as simple help text.

This could replace the help dialog in the armor sorting dialog. Here the '?' key opens a window that explains the sorting armor dialog and gives the keybindings to it. This might be useful to generalize into a common system:

'?' (or the key you bound to `HELP_KEYBINDINGS`) shows a dialog with the keybindings and at the bottom some text, that has been set up with `ctxt.set_help_text(...)`.

This could replace the existing help text of sorting armor and could be extend to other screens as well. Does this text sound redundant?