Allow focussing the main view

In this commit this is only possible by pressing '0' in a side panel; we'll add
mouse clicking later in the branch.

Also, you can't really do anything in the focused view except press escape to
leave it again. We'll add some more functionality in a following commit.

pkg/gui/controllers/main_view_controller.go

@@ -0,0 +1,63 @@
+package controllers
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/gui/types"
+)
+
+type MainViewController struct {
+	baseController
+	c *ControllerCommon
+
+	context      types.Context
+	otherContext types.Context
+}
+
+var _ types.IController = &MainViewController{}
+
+func NewMainViewController(
+	c *ControllerCommon,
+	context types.Context,
+	otherContext types.Context,
+) *MainViewController {
+	return &MainViewController{
+		baseController: baseController{},
+		c:              c,
+		context:        context,
+		otherContext:   otherContext,
+	}
+}
+
+func (self *MainViewController) GetKeybindings(opts types.KeybindingsOpts) []*types.Binding {
+	return []*types.Binding{
+		{
+			Key:             opts.GetKey(opts.Config.Universal.TogglePanel),
+			Handler:         self.togglePanel,
+			Description:     self.c.Tr.ToggleStagingView,
+			Tooltip:         self.c.Tr.ToggleStagingViewTooltip,
+			DisplayOnScreen: true,
+		},
+		{
+			Key:         opts.GetKey(opts.Config.Universal.Return),
+			Handler:     self.escape,
+			Description: self.c.Tr.ExitFocusedMainView,
+		},
+	}
+}
+
+func (self *MainViewController) Context() types.Context {
+	return self.context
+}
+
+func (self *MainViewController) togglePanel() error {
+	if self.otherContext.GetView().Visible {
+		self.otherContext.SetParentContext(self.context.GetParentContext())
+		self.c.Context().Push(self.otherContext, types.OnFocusOpts{})
+	}
+
+	return nil
+}
+
+func (self *MainViewController) escape() error {
+	self.c.Context().Pop()
+	return nil
+}