[Shell] Add Tab bar and auto promote tabs to nearest TabBar

[PureWeen]

Description of Changes

If you specify Tabs without a container then they automatically get promoted to the nearest implicit TabBar.

If you specify a type as a TabBar it auto disables the Flyout Behavior when you are on that TabBar

Example 1

<Shell>
   <Tab Title="Home" Icon="home.png">
        </Tab>
   <Tab Title="Home" Icon="home.png">
        </Tab>
</Shell>

becomes

<Shell>
     <TabBar>
        <Tab Title="Home" Icon="home.png">
         </Tab>
          <Tab Title="Home" Icon="home.png">
        </Tab>
     </TabBar>
</Shell>

Example 2

<Shell>
   <Tab Title="Home" Icon="home.png">
        </Tab>
   <Tab Title="Home" Icon="home.png">
        </Tab>
<TabBar>
   <Tab Title="Home" Icon="home.png">
        </Tab>
   <Tab Title="Home" Icon="home.png">
        </Tab>
</TabBar>
   <Tab Title="Home" Icon="home.png">
        </Tab>
   <Tab Title="Home" Icon="home.png">
        </Tab>
</Shell>

becomes

<Shell>
     <TabBar>
        <Tab Title="Home" Icon="home.png">
        </Tab>
        <Tab Title="Home" Icon="home.png">
        </Tab>
     </TabBar>
     <TabBar>
        <Tab Title="Home" Icon="home.png">
        </Tab>
        <Tab Title="Home" Icon="home.png">
        </Tab>
     </TabBar>
     <TabBar>
        <Tab Title="Home" Icon="home.png">
        </Tab>
        <Tab Title="Home" Icon="home.png">
        </Tab>
     </TabBar>
</Shell>

API Changes

• Added Tab Bar alias for ShellItem which disables the flyout by default

Visual Changes

Draft PR of being able to specify

<Shell>
   <Tab Title="Home" Icon="home.png">
            <ShellContent>
                <ContentPage></ContentPage>
            </ShellContent>
        </Tab>
        <Tab Title="Library" Icon="library.png">
            <ShellContent>
                <ContentPage></ContentPage>
            </ShellContent>
        </Tab>
        <Tab Title="Contact" Icon="contact.png">
            <ShellContent>
            <ContentPage></ContentPage>
        </ShellContent>
        </Tab>
</Shell>

to render as one page

[PureWeen]

API 28 failures are unrelated