preferences: Relabel "Reset Defaults" button in Hotkey preferences to "Defaults"

Another attempt to reduce translator suicide rates.

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56171 75b3775d-3801-0410-9e12-e2e43121521d
mp options: Use a pre-existing standards-conforming label for the "Restore defaults button"

Previously that label would have been "Reset Defaults" (hotkey
preferences), but since that's just been changed it will be "Defaults"
instead.

Please use title case for button labels.

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56172 75b3775d-3801-0410-9e12-e2e43121521d