Fix tab contrast colors when in high contrast

[carlos-zamora]

Summary of the Pull Request

Originally, the XAML resources were being applied on the TabView's ResourceDictionary directly. However, high contrast mode has a few weird scenarios as it basically reduces the color palette to just a few colors to ensure high contrast. This PR now stores the resources onto the ThemeDictionaries so that we have more control over the colors used.

References and Relevant Issues

Closes #17913
Closes #13067 (fixed by 5be0056)

Validation Steps Performed

Compared the following scenarios to WinUI 2 gallery's TabView when in High Contrast mode:
✅ (Un)selected tab
✅ hover over x of (un)selected tab
✅ hover over unselected tab

[carlos-zamora]

Demo

Selected vs unselected

Unselected (hovered)

Unselected (Hover over X)

Selected (Hover over X)

[carlos-zamora]

Discussion

The OS allows users to set a contrast theme:

This is achieved by using the following XAML resource keys:

However, Windows Terminal offers a lot of built-in customizability, namely through color schemes and theming. In fact, the default theme is:

{
    "name": "dark",
    "window": {
        "applicationTheme": "dark"
    },
    "tab": {
        "background": "terminalBackground",
        "unfocusedBackground": "#00000000"
    },
    "tabRow": {
        "unfocusedBackground": "#333333FF"
    }
},

So, the question is, how do we balance the two? What I've done here is ignore the OS theme, basically. The main one I'm looking at in the default theme is tab.background="terminalBackground" since that's a dynamic value. There may be some scenarios where high contrast may not be guaranteed. Though, the default color scheme with default theme works, and that's really the important one.

CC @zadjii-msft as the father of WT Theming

[carlos-zamora]

Discusion

Discussed as a team. The current solution works just fine. We'll keep an eye on the tab row and other reported High Contrast issues for the future.