This enables the quick access button [to show the relevant Texture in
the Properties Editor] whereever brush textures are shown:
- header 'Texture' popup
- Sidebar > Tool tab (Brush Settings > 'Texture' panel)
- Properties Editor > Active Tool tab (Brush Settings > 'Texture' panel)
This goes in line to what we do for other textures:
- modifier textures have this button
- particle textures have this button
- probably others as well
Following changes were needed:
- call template_ID_preview for brush.texture_slot.texture (rather than
brush.texture), otherwise check in uiTemplateTextureShow /
template_texture_show does not match ButsContextTexture users
- utility to find the Properties Editor in the screen

