stats: fix some issues with bomber obsolescence
data: increased Manchester accuracy a little
Made some UI changes (yellow crosshair), which necessitate SDL_gfx

SDL_gfx is used for an alpha-alpha blitter, SDL_gfxBlitRGBA, that (unlike
 SDL_BlitSurface) *actually* *works*.
Fix bug where raids remained specified on quit & load/new game
New stats report "tombstone" gives number of aircrews Missing In Action

If the figure is not zero, it also draws an ASCII-art gravestone bearing the
 legends "RIP" and "WE WILL REMEMBER THEM".  Hopefully this will come across
 as poignant and not just camp.
Allow Ctrl-click as well as middle-click to assign/deassign all
Put a proper 'Continue' button on message_box (instead of using the signature)
Add a TODO about toggleable map overlays
stats: Various fixes & a new graph, of enemy fighters
data: Slightly improved Stirling's accuracy
Added a new ftrbase.  Breaks saves!

To manually upconvert, change FBases from 11 to 12.  If you really want
 precise conversion, increment the first value after the colon in fighter
 records (ie. 'Type 0:1,blahblah' -> 'Type 0:2,blahblah')
Added navigation wobble and fuel exhaustion crash for fighters
A collection of changes to game logic & data, which together rebalance

Flak and fighters are much more deadly, but the budget is higher and damage is
 repaired more quickly (and serviceability actually equilibriates above its
 nominal value).

The intention is to try and make loss-rates more historically realistic.

Playtesting will be required to ensure that overall balance is OK.
stats: New graph with fighter kills & losses; other graphing fixes
Improve kill tracking (flak vs fighter) & tweak balance
stats/valuegraph: added --nolegend option (because it often gets in the way)
Don't send fighters to places they can't reach
Ensure a/c types' .text & .newtext are initialised to NULL
Make the window resizeable
Added button to reset window to default size (800x640)