Implement all the missing parts of the Edit Sheets and Edit Sounds dialogs

- On load, the game now detects graphic sheets and sounds whose IDs are "discontinuous", as well as graphics intended to directly replace preset graphic sheets.
- Edit sheets dialog can now handle "discontinuous" graphics. (The edit sounds dialog already could.)
- Edit sheets dialog prompts user to create a new sheet if there are none already, and also if there are some but not ID 0 (in the latter case they can cancel and still edit the sheets).
- Edit sheets dialog prompts user to convert sheets if the scenario is legacy, rather than doing it silently
- Edit sheets dialog now has "new" and "delete" buttons
- Edit sounds dialog now has functioning "delete" button

src/dialogxml/pict.cpp

@@ -493,6 +493,7 @@ void cPict::recalcRect() {
 			break;
 		case PIC_FULL:
 		case PIC_CUSTOM_FULL:
+			if(drawScaled) break;
 			auto sheet = getSheet(SHEET_FULL, picNum);
 			sf::Vector2u sz = sheet->getSize();
 			bounds.width() = sz.x;