.vhs's ambush maps

[Xymanek]

TODO:

• [x] Validate that every plot works - during initial testing some worked but not all
• [x] Validate that (existing) images get properly shown in the save list
• [x] Validate that stuff works using an actual campaign
• [x] Rebalance spawn and enemy placement
• [x] Change kismet to spawn the evac right on the objective spot, instead of within 12 tiles

Closes #558 (in theory, since this is a less hacky approach)

[Xymanek]

Current status:

• TWN plots have issues with seamless connections of different pieces. Mostly happens around edges (but also noticed somewhere in a road once) and it's especially bad in the snowy biome
• TUN plots are simply unfinished
• Plot_WLD_Escape_01 is borked, rest of WLD works
• Plot_WLD_Escape_03 has just a single bridge bottleneck, this might be too imbalanced/punishing compared to the other maps
• Save images work, didn't test each and every though
• There is a significant space between spawn and first preplaced encounter (which are kinda bunched up on the other side of the map) which might be too simple without the lost spawning but it's also the area where the player will be fighting the RNFs. Playtesting is required
• PCPs have ObjectiveTags[1]="GatherSurvivors" in addition to CovertEscape