Fix issues with commit message panel losing focus

[jesseduffield]

This fixes a bug where if you're focused on the commit message panel, then say an error panel pops up, the commit message panel goes to the back of the views but then focus is returned to it

Now we just bring that panel to the front when it's focused.

But while fixing this bug I also decided to introduce the idea of not ever returning focus to 'second class panels' which currently just contains the confirmation panel.

Can be tested with this:

diff --git a/pkg/gui/gui.go b/pkg/gui/gui.go
index e3cc615..2e98f16 100644
--- a/pkg/gui/gui.go
+++ b/pkg/gui/gui.go
@@ -290,6 +290,11 @@ func (gui *Gui) layout(g *gocui.Gui) error {
                        return err
                }

+               go func() {
+                       time.Sleep(5 * time.Second)
+                       gui.createErrorPanel(g, "test")
+               }()
+
                // these are only called once (it's a place to put all the things you want
                // to happen on startup after the screen is first rendered)
                gui.Updater.CheckForNewUpdate(gui.onBackgroundUpdateCheckFinish, false)

[codecov-io]

Codecov Report

Merging #295 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #295      +/-   ##
==========================================
+ Coverage   93.61%   93.63%   +0.01%     
==========================================
  Files          12       12              
  Lines        1628     1633       +5     
==========================================
+ Hits         1524     1529       +5     
  Misses        101      101              
  Partials        3        3

Impacted Files	Coverage Δ
pkg/utils/utils.go	76.47% <100%> (+1.21%)	⬆️

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