Implement multi-tile placement

[zpervan]

In order to have a more pleasant experience while creating maps, implement a feature that will allow the user to place multiple tiles at once.

Workflow:

1. Select a tile
2. Position the tile and hold the left mouse button
3. Drag to the end position
4. Release the mouse button

Example:

[zpervan]

Simple muiltitile placement implemented with commit cd7ec11