let user set the side_panel height

[yoelk]

This feature is useful for apps that need to be run both on mobile
devices and on PCs. On the PC, the whole screen height might not be
needed, which leaves dead empty space in the current version.
With this new feature, the default height is the parent’s height, and
it can be explicitly set to be different by setting the
side_panel_height property to a custom value.

[KeyWeeUsr]

I wonder if it's necessary after PR #8. This will set height, but will collide with the whole widget's area even after the 8th PR. You can accomplish this thing(and with correct collisions) when you place the drawer into something as a casual widget, so it'll be like this: