Add shortcuts for filtering files by status (#3137)

- 's' for showing only staged files
- 'u' for showing only unstaged files
- 'r' for resetting the filter

- **PR Description**

- **Please check if the PR fulfills these requirements**

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[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#code-formatting))
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I've thought about adding `disabled` reasons for them but they kinda
don't make sense since if there are only `unstaged` files, disabling the
`Show only staged files` makes no sense because the `Show only unstaged
files` shows nothing different so we should disable that too, and then
what's the point of having a menu if everything except for `reset` is
disabled.

pkg/gui/controllers/files_controller.go

@@ -670,18 +670,21 @@ func (self *FilesController) handleStatusFilterPressed() error {
 				OnPress: func() error {
 					return self.setStatusFiltering(filetree.DisplayStaged)
 				},
+				Key: 's',
 			},
 			{
 				Label: self.c.Tr.FilterUnstagedFiles,
 				OnPress: func() error {
 					return self.setStatusFiltering(filetree.DisplayUnstaged)
 				},
+				Key: 'u',
 			},
 			{
 				Label: self.c.Tr.ResetFilter,
 				OnPress: func() error {
 					return self.setStatusFiltering(filetree.DisplayAll)
 				},
+				Key: 'r',
 			},
 		},
 	})