New demo build v4a.200723

• Fixed a critical bug of tower construction backtracking.
• Added crystal mining indicator for miners.
• Added a mechanism to skip the timer if there are no creeps on the map.
• Added menu button (pause).
• Added dynamic attack radius field when improving towers.
• Removed invalid improvement buttons.
• Golems now appear no earlier than level 10.
• Fixed a bug where ground towers could attack flying units on spawn.
• Minor balance tweaks.