What the title says. 😄
Rather than relying on the potentially overloaded `!=` or `==` operators
when checking for null, now we'll use the `is` expression (possibly
combined with the `not` operator) to ensure correct checking. Probably
overkill for many of these classes, but decided to err on the side of
consistency. Would matter more on classes that may be inherited or
extended.
Using `is` and `is not` will provide us a guarantee that no
user-overloaded equality operators (`==`/`!=`) is invoked when a
`expression is null` is evaluated.
In code form, changed all instances of:
```c#
something != null
something == null
```
to:
```c#
something is not null
something is null
```
The one exception was checking null on a `KeyChord`. `KeyChord` is a
struct which is never null so VS will raise an error when trying this
versus just providing a warning when using `keyChord != null`. In
reality, we shouldn't do this check because it can't ever be null. In
the case of a `KeyChord` it **would** be a `KeyChord` equivalent to:
```c#
KeyChord keyChord = new ()
{
Modifiers = 0,
Vkey = 0,
ScanCode = 0
};
```
Closes#40979
Usually, you're supposed to try to cast the action to a specific
type, and use those objects to get the data you need.
However, there's something weird with AdaptiveCards and the way it
works when we consume it when built in Release, with AOT (and
trimming) enabled. Any sort of `action.As<IAdaptiveSubmitAction>()`
or similar will throw a System.InvalidCastException.
Instead we have this horror show.
The `action.ToJson()` blob ACTUALLY CONTAINS THE `type` field, which
we can use to determine what kind of action it is. Then we can parse
the JSON manually based on the type.
Refactored ContextMenu into it's own control to allow displaying in
CommandBar and in response to right click on list items.
- Adds "critical" styling to context menu items flagged as `IsCritical`.
This will use the theme to style with correct color.
- Added `SeparatorContextItem` and modified `MoreCommands` to allow for
both `CommandContextItem`s and `SeparatorContextItem`s.
- Right clicking a list item with a context menu will open the context
menu at the position of the click and position the filter box at the top
of the context menu.


This PR covers:
- closes#38308
- closes#39211
- closes#38307
- closes#38261
_targets #38573_
At first I just wanted to add support for nested context menus.
But then I also had to add a search box, so the focus wouldn't get weird.
End result:

This gets rid of the need to have the search box and the command bar both track item keybindings - now it's just in the command bar.
Closes#38299Closes#38442
* [x] Re-adds the context menu shortcut text
* [x] Hooks up the keybindings to the search box so that you can just press the keys while you have an item selected, and do a context command
* [x] Hook these keybindings up to the context flyout itself
* [x] Adds a sample for testing
Solves #38271
**WARNING:** This PR will probably blow up all in-flight PRs
at some point in the early days of CmdPal, two of us created seperate
`Exts` and `exts` dirs. Depending on what the casing was on the branch
that you checked one of those out from, it'd get stuck like that on your
PC forever.
Windows didn't care, so we never noticed.
But GitHub does care, and now browsing the source on GitHub is basically
impossible.
Closes#38081