Refactor style apply

[grokys]

What does the pull request do?

Refactors how we attach styles:

• Removes the IStyler service and the Styler implementation
  • Having this as a pluggable service was only really done for unit testing purposes, but it meant that we had to do a service lookup each time a control wanted to style itself
• Moves the logic for applying styles and control themes from Styler into StyledElement
• Removes the style TryAttach method from the public API
  • I want to allow the possibility of refactoring this code later without breaking a public API

Also adds a fix for #8549:

• Don't apply styling in TopLevel constructor. This isn't a good idea for a number of reasons, one of which is demonstrated by Window gets :is(Control) style applied twice #8549
• Instead apply styling when showing a window, or if it's needed call StyledElement.ApplyStyling manually, e.g. in unit tests.

Performance Regression

This PR removes style caching for now. This will need to be added back in future, but:

• The old method of caching styles inside individual Style/Styles objects wasn't very efficient. We'll need to implement something better, that also helps speed up applying control themes
• Now we have control themes we have a lot less styles than when style caching was added so probably less important

I modified a benchmark in this PR to test this; it adds a lot of styles for various controls in a nested tree and instantiates and styles a single TextBox:

Before:

Method	Mean	Error	StdDev	Gen 0	Allocated
Apply_Detach_Styles	68.18 us	1.275 us	1.065 us	3.7842	15 KB

After:

Method	Mean	Error	StdDev	Gen 0	Allocated
Apply_Detach_Styles	109.4 us	0.89 us	0.84 us	4.2725	17 KB

So we have a pretty serious performance regression in the worst case, though this only affects applications with a lot of styles. For typical applications which use mostly control themes it shouldn't be noticeable.

I don't want to address this in this PR as it's a separate feature, but will open a PR to add back style caching using a new method in the near future, which should improve things beyond what we had before.

Depends on #8600
Fixes #8549

[avaloniaui-team]

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[avaloniaui-team]

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[avaloniaui-team]

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[avaloniaui-team]

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