Made texture browser category levels customisable

Controlled by the cvar 'map_tex_treespec', a comma-separated combination of 'archive', 'type' and 'category'
Fix incorrect scale values when copying textures

Fixes #130
Add recent file on 'Save As'
Added option to auto-apply property changes

Controlled by cvar 'mobj_props_auto_apply', when set removes the need to click the 'apply' button to save property changes. Closes #101
Merge branch 'master' of https://github.com/sirjuddington/SLADE into Music