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/switch_to_focused_main_view_controller.go

@@ -0,0 +1,49 @@
+package controllers
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/gui/types"
+)
+
+// This controller is for all contexts that can focus their main view.
+
+var _ types.IController = &SwitchToFocusedMainViewController{}
+
+type SwitchToFocusedMainViewController struct {
+	baseController
+	c       *ControllerCommon
+	context types.Context
+}
+
+func NewSwitchToFocusedMainViewController(
+	c *ControllerCommon,
+	context types.Context,
+) *SwitchToFocusedMainViewController {
+	return &SwitchToFocusedMainViewController{
+		baseController: baseController{},
+		c:              c,
+		context:        context,
+	}
+}
+
+func (self *SwitchToFocusedMainViewController) GetKeybindings(opts types.KeybindingsOpts) []*types.Binding {
+	bindings := []*types.Binding{
+		{
+			Key:         opts.GetKey(opts.Config.Universal.FocusMainView),
+			Handler:     self.handleFocusMainView,
+			Description: self.c.Tr.FocusMainView,
+		},
+	}
+
+	return bindings
+}
+
+func (self *SwitchToFocusedMainViewController) Context() types.Context {
+	return self.context
+}
+
+func (self *SwitchToFocusedMainViewController) handleFocusMainView() error {
+	mainViewContext := self.c.Helpers().Window.GetContextForWindow("main")
+	mainViewContext.SetParentContext(self.context)
+	self.c.Context().Push(mainViewContext, types.OnFocusOpts{})
+	return nil
+}