Add `tabs` to `layouts`

fixes #603, fixes #349

* The layout has now a unique `tabs` section,
  that can be used, like the `parts` section,
  everything that is not inside the tabs section
  is assumed to be present on every single tab
  that is opened.

  This is a BREAKING CHANGE for people that use
  custom `layouts` already, since the `tabs` section
  is not optional - for clarity and intentionality reasons.

  The functionality to specify multiple tabs is already there,
  but is still gated behind a panic, until #621 is fixed.
  So for now one tab can be specified to load on startup.

* The `NewTab` action can optionally be bound to open
  a layout that is assumed to be in the new `tabs` section

  This is a BREAKING CHANGE for people that have the
  `NewTab` action already bound in the config file:

```
- action: [NewTab, ]
  key: [F: 5,]
```
must now be specified as:
```
- action: [NewTab: ,]
  key: [F: 5,]
```
  Optionally a layout that should be opened on the new tab can be
  specified:
```
- action: [NewTab: {
  direction: Vertical,
  parts: [ {direction: Horizontal, split_size: {Percent: 50}}, {direction: Horizontal, run: {command: {cmd: "htop"}}},],
  key: [F: 6,]
```
or:
```
- action: [NewTab: {direction: Vertical, run: {command: {cmd: "htop"} }},]
  key: [F: 7,]
```
or
```
- action: [NewTab: {
  direction: Vertical,
  parts: [ {direction: Vertical, split_size: {Percent: 25},run: {plugin: "strider" }}, {direction: Horizontal}],}, MoveFocus: Left,]
  key: [F: 8,]

```

1 files changed, 2 insertions, 1 deletions

diff --git a/src/main.rs b/src/main.rs
index b3706ead..050c0cca 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -94,7 +94,8 @@ pub fn main() {
                 opts.layout.as_ref(),
                 opts.layout_path.as_ref(),
                 layout_dir,
-            );
+            )
+            .map(|layout| layout.construct_main_layout());
 
             start_client(
                 Box::new(os_input),