feat: add focus attribute in layout

[jaeheonji]

This PR resolve #852 , resolve #942

Description

The focus attribute is added to the layout like this:

---
template:
  ~
tabs:
- direction: Vertical
- direction: Vertical
  focus: true # this
  parts:
    - direction: Vertical
      split_size:
        Percent: 50
    - direction: Vertical
      focus: true # this
      split_size:
        Percent: 50

First, each TabLayout has a focus attribute. The focus value of the top-level TabLayout determines "which tab to focus on."
The sub-level TabLayout refers to the pane of each tab, it determines "which pane to focus on".

If focus attribute is duplicated, tab or pane with the first declared value is focused.

Also, to keep the state of the last focused pane within each tab, the Tab struct has a last_active_pane_id variable.
This saves the state of the focused pane from the layout, and at the same time preserves the state of the previous pane when the tab is moved. (Previously, it was initialized to the first (Index 0) pane)

I declared the last_active_pane_id variable as an Option in case there was any problem with the system. If it's no problem, I will remove the Option type.

Other thing..

To focus the tab, internally call GoToTab Instruction after all layouts are loaded.
When zellij starts with multiple tabs layout, it currently has the following process.

newTab -> render -> newTab -> render -> newTab -> render ... -> GoToTab -> render

Of course, this is not a big problem but,

newTab -> newTab -> newTab -> ... -> GoToTab -> render

It might be a good idea to render only once, only when the program is started. Of course this is a different issue.

EDIT: I will update the test-related errors when this feature is determined to be ok.

[djpate]

That's exactly what I need. +1

[djpate]

If this was passing specs, would you consider merging this? @imsnif @tlinford

[imsnif]

If it passes CI and @jaeheonji thinks it's good, I'm happy.

[jaeheonji]

@djpate Thanks for your interest!

@imsnif The original concept for this feature was presented by @a-kenji, so I respect his opinion and await a review.
However, since he was already agreed on the overall spec (although review has not been done yet), I'll merge this features after CI and strict testing.

As an aside, I'm currently busy until this week, so please be patient if the work is a little late 🙏

[imsnif]

@jaeheonji - there is no rush! Please take your time.

[a-kenji]

Sorry that it took this long.
This is awesome!
LGTM!

[jaeheonji]

History Note:

I did not fully implement the function I initially expected.

In particular, the ability to save the state of the focused Pane in each Tab is not implemented at this time due to several problems.

(Because I found a conflict with the multiple user function and an unexpected bug internally. This should be managed as a separate issue.)

So, I only implement focusing on specific Tab and Pane through Layout.