Merge pull request #132 from Gaerzi/master

Fix a couple of obscure bugs.
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
Add mobj_props_auto_apply option to preferences dialog
Properly detect folders dragged to the start page
Bump version to beta 4
Add undo history panel to map editor window

- Shortcut key Ctrl+3 (script editor changed to Ctrl+5 to keep things consistent with the main window)
- Add 'Time' column to undo history panel