New Achievements, Tidal Power, Export Targets

This update contains the following changes:

• Added Export Targets option to custom game menu (requires an increasing number of export contract completions per cycle)
• The Snow Town achievement has been replaced by the Ice King achievement
• Added Speedster achievement
• Added Tidal Power Plant
• Fixed broken text for train temperament selectors

The Export Targets rule is intended to allow increasing the tension in custom games, especially when playing with "Allow actions while paused". In the future, I'm planning to add a few presets in a submenu under "Start Game", some of which will include this rule.

The Snow Town achievement was replaced due to being too dependent on random numbers. The Ice King achievement requires the Train Core to be above the snow line instead of the Town Hall. This makes a somewhat similar type of challenge, but much more likely to encounter a map where it is possible.

The Speedster achievement requires doing an unusual playstyle where only "fast" trains are used, requiring special considerations, especially to get access to them at the start of the game.

In Distributrains, the sea has just been an obstacle, with no real uses. The Tidal Power Plant changes this by allowing power to be extracted from water connected to the map edge. It consumes metal to build tidal generators, which produce constant output independent of climate. However, these break down after a time (e.g. due to being in a highly corrosive environment), and cost metal to be rebuilt, so this power plant is suitable for metal-rich runs.