Pressing Shift while painting with Stamp Brush should start line mode #3642

When trying to use Shift + Stamp Brush to draw lines, I often end up clicking first and then pressing Shift because that feels more natural to me, but in Tiled this doesn't work - it does not begin line-drawing and causes any further cursor movement to do nothing until I let go of the mouse button.

I think it would be more convenient if pressing Shift while already painting would start drawing a line from the cursor's location at that point. If the user releases Shift before the mouse button, then the line should not be drawn, same as the current line-drawing behaviour, but if they're still holding the mouse, they should start painting normally from the current location (Edit: thinking about it more, it should not keep drawing without clicking again, as it's likely if the user changes their mind about painting a line, their cursor isn't somewhere where they want to immediately paint. My suggestion here requires too much preplanning of the user xP).