lib/ui: fix Grid race condition

This was is more complicated than others. The cells list is accessed by
multiple goroutines:

- Some change the Grid's contents via AddChild/RemoveChild
- Some call Draw
- Some invalidate the grid via Invalidate

Invalidate calls are tricky to handle because they will also invalidate all
child cells. This will inturn trigger the cellInvalidated callback, which needs
to read the list of cells.

For this reason, we use a sync.RWLock which allows multiple concurrent reads.

Below is the race fixed by this commit.

    Read at 0x00c0000bc3d0 by goroutine 7:
      git.sr.ht/~sircmpwn/aerc2/lib/ui.(*Grid).cellInvalidated()
          /home/simon/src/aerc2/lib/ui/grid.go:181 +0x45
      git.sr.ht/~sircmpwn/aerc2/lib/ui.(*Grid).cellInvalidated-fm()
          /home/simon/src/aerc2/lib/ui/grid.go:179 +0x55
      git.sr.ht/~sircmpwn/aerc2/lib/ui.(*Invalidatable).DoInvalidate()
          /home/simon/src/aerc2/lib/ui/invalidatable.go:22 +0x85
      git.sr.ht/~sircmpwn/aerc2/lib/ui.(*Bordered).contentInvalidated-fm()
          /home/simon/src/aerc2/lib/ui/borders.go:39 +0x56
      git.sr.ht/~sircmpwn/aerc2/lib/ui.(*Invalidatable).DoInvalidate()
          /home/simon/src/aerc2/lib/ui/invalidatable.go:22 +0x85
      git.sr.ht/~sircmpwn/aerc2/widgets.NewDirectoryList.func1()
          /home/simon/src/aerc2/widgets/dirlist.go:81 +0x55
      git.sr.ht/~sircmpwn/aerc2/lib/ui.(*Invalidatable).DoInvalidate()
          /home/simon/src/aerc2/lib/ui/invalidatable.go:22 +0x85
      git.sr.ht/~sircmpwn/aerc2/widgets.(*Spinner).Start.func1()
          /home/simon/src/aerc2/widgets/spinner.go:88 +0x82

    Previous write at 0x00c0000bc3d0 by main goroutine:
      [failed to restore the stack]

    Goroutine 7 (running) created at:
      git.sr.ht/~sircmpwn/aerc2/widgets.(*Spinner).Start()
          /home/simon/src/aerc2/widgets/spinner.go:46 +0x98
      git.sr.ht/~sircmpwn/aerc2/widgets.NewDirectoryList()
          /home/simon/src/aerc2/widgets/dirlist.go:37 +0x28b
      git.sr.ht/~sircmpwn/aerc2/widgets.NewAccountView()
          /home/simon/src/aerc2/widgets/account.go:49 +0x5ca
      git.sr.ht/~sircmpwn/aerc2/widgets.NewAerc()
          /home/simon/src/aerc2/widgets/aerc.go:60 +0x807
      main.main()
          /home/simon/src/aerc2/aerc.go:65 +0x33e

0 files changed, 0 insertions, 0 deletions