Improve keybinding descriptions

This adds a bunch of tooltips to keybindings and updates some keybinding descriptions (i.e. labels).

It's in preparation for displaying more keybindings on-screen (in the bottom right of the screen),
and so due in part to laziness it shortens some descriptions so that we don't need to manage both
a short and long description (for on-screen vs in-menu). Nonetheless I've added a ShortDescription
field for when we do want to have both a short and long description.

You'll notice that some keybindings I deemed unworthy of the options view have longer descriptions,
because I could get away with it.

pkg/gui/controllers/context_lines_controller.go

@@ -45,11 +45,13 @@ func (self *ContextLinesController) GetKeybindings(opts types.KeybindingsOpts) [
 			Key:         opts.GetKey(opts.Config.Universal.IncreaseContextInDiffView),
 			Handler:     self.Increase,
 			Description: self.c.Tr.IncreaseContextInDiffView,
+			Tooltip:     self.c.Tr.IncreaseContextInDiffViewTooltip,
 		},
 		{
 			Key:         opts.GetKey(opts.Config.Universal.DecreaseContextInDiffView),
 			Handler:     self.Decrease,
 			Description: self.c.Tr.DecreaseContextInDiffView,
+			Tooltip:     self.c.Tr.DecreaseContextInDiffViewTooltip,
 		},
 	}