Implement disabled state for Checkbox in Tree

[DarkMessiah]

Add disabled icons for Checkbox in Tree

Editor Theme	Base Theme

EDIT: Added icon for Indeterminate Disabled state.
EDIT2: Fixed the base theme.

[KoBeWi]

P.S. Do we need disabled state for intermediate Checkbox?

I'd say yes. Unless there is no way to set both disabled and indeterminate.

[seppoday]

Little suggestion for discussion. If we also change disabled font color it will be easier to differentiate "Unchecked" and "Unchecked Disabled". Right now, they are pretty much the same.

Difference could be more subtle than what I've shown. I've used same grey as on disabled checkbox.

[DarkMessiah]

That looks nice! Thanks for suggestion. I can add font_disable_color for Tree and use it only for check mode.

@KoBeWi What do you think?

[KoBeWi]

Makes sense I think.

[DarkMessiah]

Added font_disabled_color. It is only used for a CELL_MODE_CHECK mode cell when it's non-editable.

[DarkMessiah]

Fixed the base theme. Optimized indeterminate.svg and added indeterminate_disabled.svg

[YuriSizov]

Thanks!