Allow users to resize PalettePanel y.size to zero to make is consistent with ToolSettingPanel.

[yuyutsupoudel]

Pixelorama version:

latest

OS/device including version:

popos

Issue description:

can drag tool setting window to zero to expand palette window to maximum but can not do vice versa. If user can expand palette window to take full space, user should also be able to expand tool setting windows to maximum.

Steps to reproduce: https://user-images.githubusercontent.com/49397015/122625306-3e296600-d0c4-11eb-9e0b-1209e6d1f3d8.mp4