Improved signal placement — Distributrains Steam update #1620555

Added a signal placement mode which places signals along the shortest track path between two points Signals now use a small amount of power Trains will no longer lose their identity if moved to an empty top slot using drag and drop, or moved to the bottom when they have only one vehicle. Fixed bugs where the UI would sometimes become temporarily unresponsive to mouse input, such as after dragging out from the resource selector in the train orders window Disabled rail modifiers for tutorials where they are not meant to be used Fixed a potential memory error if the player used signals in the first two tutorials Hopper descriptions no longer neglect to mention Waste Fixed potential memory corruption caused by cloning a train and then destroying the target structure for an order inherited from the original train More progress on high dpi icons