[Feature]: Ability to set dimention of windows in other forms than pixels

[VimYoung]

It is necessary to be able to define these values dynamically so that dimensions are changed based on monitor specs. Following forms are proposed.

1. By pixels (hardcoded dimension specs currently supported).
2. By percentage width or height of monitor (proposed)
3. Defining zeros so that it can stretch based on values of anchors.

The issue with the implementation is that the slint skia renderer requires the information of dimension at an early stage. Something, the later approaches doesn't specify immediately.