Add Input Method support to the Platform API

[tronical]

Currently all IME related functions are in WindowAdapterInternal, but we should make the API public to allow others implementing platform backends to provide or communicate with input methods.

We discussed the following API:

1. A new function in WindowAdapter:

pub trait WindowAdapter {
    // Update the state of the input method
    fn input_method_request(&self, request: InputMethodRequest) {}
}

pub enum InputMethodRequest {
    Enable(InputMethodProperties),
    Update(InputMethodProperties),
    Disable
}

#[non_exhaustive]
#[derive(Clone, Default)]
pub struct InputMethodProperties {
    /// Surrounding text: (does not include pre-eedit text / composition)
    pub text: SharedString,

    pub cursor_position: usize,
    pub anchor_position: Option<usize>,
    pub preedit_text: SharedString,
    /// When the preedit_text is not empty, this is the offset of the preedit within the `text`
    pub preedit_offset: usize
    /// in window coordinates
    pub cursor_rect_origin: LogicalPosition,
    pub cursor_rect_size: LogicalSize,
    pub input_type: slint::platform::InputType, 

}

2. For updating the state based on data from the IME, we add a new event:

pub enum WindowEvent {
    InputMethod(InputMethodEvent)
}

#[non_exhaustive]
#[derive(Default)]
struct InputMethodEvent {
    /// When None, replaces the selection
    pub replacement_range: Option<Range>,
    pub commit_text: Option<SharedString>,
    pub preedit_text: SharedString,
    /// When None, the cursor is pur after the commit_text
    pub cursor_position: Option<usize>,
}

The data structures are deliberately non-exhaustive to permit adding new fields in the future.

[ogoffart]

The API has been implemented (in Rust) but is still internal