Fix problem with Drawing::drawTextureWithin
Add simple NumberTextCtrl class for numeric input

Also allows '++' and '--' prefixes for increment/decrement (will be used on property panels)
Add custom sector properties dialog, among other things:

- Split out SectorSpecialDialog stuff to SectorSpecialPanel class
- Rework properties panels a bit, now all based on PrefsPanelBase class
- Replace text controls for numbers in SidePropsPanel with NumberTextCtrls
Some improvements to line/sector/3d info overlays

- Show question mark icon instead of missing texture for unknown textures/flats
- Show required textures that are missing in red+bold text on the line info overlay
Fix text wrapping for arg descriptions on ArgsPanel
Use unknown texture icons on side/sector property panels
More changes to missing/unknown textures on overlays

- Add new 'minus' thing icon
- Show minus icon for missing textures on line and 3d info overlays
- Draw unknown/invalid textures as icon only w/red text on line and sector info overlays
Merge branch 'master' of https://github.com/sirjuddington/SLADE into osx