feat(flat-components): add avatar window

[hyrious]

Notes about the implementation @netless-io/native

New synced storage userWindows:

connectStorage("userWindows", {
  mode: 'normal',         // normal | maximized
  [userUUID]: {
    x, y, width, height,
    z,                    // window z-index in normal mode
    index,                // order in maximized mode
  }
})

Where x, y, width, height are all in the range of 0 to 1.

Windows have constant aspect ratio = $\frac{3}{4}$ (the same as video avatar).

The default window size is (0.4, 8/15):

$$\begin{align*} \texttt{width} &= 0.4 \\\ \texttt{height} &= \texttt{width} \cdot \frac{\frac{3}{4}}{\frac{9}{16}} \\\ &= \frac{8}{15} \end{align*}$$

The minimal window size is width = $0.25$.

When create a new window, set its index = maxIndex + 1, z = maxZ + 1.

When click or move a window, set its z = maxZ_except_itself + 1.

When use double click to create a window, set its x = y = 0.1, width = 0.4, height = 8/15.