Fix #52: Don't require double click to destroy buildings.

There is already a safety measure in the dialog box that needs to be confirmed before a building is destroyed. In addition, this fix allows the keyboard shortcut to activate the destroy button.
Fix #46: Replace problematic save path characters.

Add function to replace certain characters in a string (strreplace).
Replace problematic characters in file names. These are primarily a problem on windows platforms, but also other platforms using for example the FAT file system.
Fix #42: Print missing percentage sign in knight morale popup.
Fix #48: Remove visible cursor trail.

Store a buffer with the surface content potentially occluded by the cursor. Using this buffer the old cursor can be undrawn before it is moved.
Fix: playback of some sfx associated with working building.
building: Add BUILDING_IS_ACTIVE() macro to check activity bit.