Add Tool Escape / Unescape feature.

Like in the polish feature of the Add Move Selection feature (https://github.com/bjorn/tiled/issues/650), the Escape key should be usable to return to the previously used Tool mode in the main window.

For example, the user may go into Rectangular Selection mode, highlight some tiles, and then press Ctrl+C to copy. When the user hits Ctrl + V to Paste, Tiled enters into the Stamp Brush mode. Hitting the Escape key should return it Tiled back to Rectangular Selection mode.

Further, if a different Tool mode was active before the user entered into Rectangular Selection mode, an additional press of the Escape key should return them to that mode, and so on.

Yet further, Shit+Escape should preform the same type of navigation, but in the opposite direction. So if the user pressed Escape to go from Stamp Brush mode to Rectangular Selection mode, Shift+Escape should return them to Stamp Brush mode. So on forward for multiple presses of Shift+Escape.

Start at the main window's tool-mode handling and trace how Rectangular Selection, Stamp Brush, and Paste change the active tool. Verify that Escape moves backward through prior tools, Shift+Escape moves forward, and repeated presses preserve the navigation history as described.