Allow fill terrain modifier hotkey to apply to raise/lower cells

[Starkku]

Allows holding the Fill Terrain hotkey while using the Raise / Lower Individual Cells tools to apply the action to an enclosed area similar to filling an area of cells with 1x1 tiles.

I was initially going to add its own customizable hotkey for this, but for sake of convenience and consistency I would have wanted both actions to use the same key, and current system does not allow mapping same key to multiple actions (for obvious reasons). So it was either this or hardcoding it to holding Ctrl or something.

[Starkku]

Moved the helper function to Helpers.cs.

[Starkku]

Added the missing height checks.