Add support for slint files

[qarmin]

Files with extension .slint are used by slint framework e.g. - https://github.com/slint-ui/slint/blob/v1.3.2/examples/slide_puzzle/slide_puzzle.slint

struct Piece  {
    // col/row position of the tile in the puzzle
    pos-x: int,
    pos-y: int,
    // offset in pixel from the base position for the kicking animation
    offset-x: length,
    offset-y: length,
}

struct Theme  {
    name: string,
    window-background-color: brush,
    game-background-color: brush,
    game-use-background-image: bool,
    game-border: length,
    game-radius: length,
    game-text-color: color,
    game-highlight-color: color,
    piece-border: length,
    piece-background-1: brush,
    piece-background-2: brush,
    piece-border-color-1: brush,
    piece-border-color-2: brush,
    piece-text-color-1: color,
    piece-text-color-2: color,
    piece-text-weight-incorrect-pos: int,
    piece-text-weight-correct-pos: int,
    piece-text-font-family: string,
    piece-radius: length,
    /// Ratio of the piece size
    piece-spacing: float,
}

[zhangzqs]

#1054