[3.x] Backport non-breaking changes in the Theme resource from master

[YuriSizov]

Following changes has been ported:

1. Add missing type list methods (Remove unused argument in Theme method and expose missing methods #37759) (named *_types to follow get_stylebox_types, renamed in master):

   • Theme.get_color_types() (get_color_type_list in master)
   • Theme.get_constant_types() (get_constant_type_list in master)
   • Theme.get_font_types() (get_font_type_list in master)
   • Theme.get_icon_types() (get_icon_type_list in master)

2. Add utility methods with all fixes applied (Add utility methods to Theme, improve error messages and documentation #47251, Fix Theme overriding existing theme types with empty #49308):

   • All as described in Add utility methods to Theme, improve error messages and documentation #47251, except for font size related methods as it doesn't exist in 3.x

3. While Add theme_custom_type property to Control and Window #47544 is not portable due to compatibility breaking fixes, one fix can be ported as it is just a typo.

All changes are split into individual commits related to each backported changeset/PR.

[akien-mga]

Thanks!