Textures Browser form: empty texture sets are no longer shown when mixed...

Textures Browser form: empty texture sets are no longer shown when mixed textures & flats is disabled in the current game configuration.
Textures Browser form: PK3/Directory TEXTURES images are now shown in a separate folder in the resources tree.
Fixed, Textures Browser form: fixed a logic error when trying to select initial flat when mix textures & flats was disabled in the current game configuration (this resulted in the blank textures list after opening the form).
Fixed, Textures Browser form: resources tree showed textures count even when browsing flats.
Fixed, Textures Browser form: PK3/Directory textures took precedence even when browsing flats (this means when there were a flat and a texture with the same name, a texture was displayed when browsing flats).
Fixed, Classic modes: actor's scale set in DECORATE was ignored when rendering models.
Fixed, MODELDEF parser: in some cases, several model definitions were skipped when trying to skip the current one.
Fixed, resource management: flat and sprite TEXTURES definitions were loaded only from TEXTURES files named "TEXTURES".
Fixed/added, PK3/folder resource management: patch locations for sprites defined in TEXTURES are now checked the same way as in ZDoom (previously only the "sprites" folder was checked).
Fixed/added, PK3/folder resource management: patch locations for textures defined in TEXTURES are now checked the same way as in ZDoom (previously only the "textures" folder was checked).
Fixed, PK3/folder resource management: flats defined in TEXTURES were not added to the global Flats image list.
Fixed, PK3/folder resource management: in some cases, the image search algorithm could find flats, textures, patches or sprites in incorrect folders (for example, it could find a flat in "flats_backup" folder).