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