settings_ui: Move settings UI trait to file content (#37337)

Closes #ISSUE

Initially, the `SettingsUi` trait was tied to `Settings`, however, given
that the `Settings::FileContent` type (which may be the same as the type
that implements `Settings`) will be the type that more directly maps to
the JSON structure (and therefore have the documentation, correct field
names (or `serde` rename attributes), etc) it makes more sense to have
the deriving of `SettingsUi` occur on the `FileContent` type rather than
the `Settings` type.

In order for this to work a relatively important change had to be made
to the derive macro, that being that it now "unwraps" options into their
inner type, so a field with type `Option<Foo>` where `Foo: SettingsUi`
will treat the field as if it were just `Foo`, expecting there to be a
default set in `default.json`. This imposes some restrictions on what
`Settings::FileContent` can be as seen in 1e19398 where `FileContent`
itself can't be optional without manually implementing `SettingsUi`, as
well as introducing some risk that if the `FileContent` type has
`serde(default)`, the default value will override the default value from
`default.json` in the UI even though it may differ (but it should!).

A future PR should probably replace the other settings with `FileContent
= Option<T>` (all of which currently have `T == bool`) with wrapper
structs and have `KEY = None` so the further niceties
`derive(SettingsUi)` will provide such as path renaming, custom UI, auto
naming and doc comment extraction can be used.

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/git_ui/src/git_panel_settings.rs

@@ -36,7 +36,7 @@ pub enum StatusStyle {
     LabelColor,
 }
 
-#[derive(Clone, Default, Serialize, Deserialize, JsonSchema, Debug)]
+#[derive(Clone, Default, Serialize, Deserialize, JsonSchema, Debug, SettingsUi)]
 pub struct GitPanelSettingsContent {
     /// Whether to show the panel button in the status bar.
     ///
@@ -77,7 +77,7 @@ pub struct GitPanelSettingsContent {
     pub collapse_untracked_diff: Option<bool>,
 }
 
-#[derive(Deserialize, Debug, Clone, PartialEq, SettingsUi)]
+#[derive(Deserialize, Debug, Clone, PartialEq)]
 pub struct GitPanelSettings {
     pub button: bool,
     pub dock: DockPosition,