point out missing mass more clearly

[nczempin]

• make a sound when someone tries to place a tower when mass is insufficient.
• "flash" the mass display or otherwise notify the player
• disable (grey out?) the towers when not enough mass is available. when the player clicks on one anyway, some notification could be shown to point to the flashing(?) mass display

These ideas could be split up into separate issues.

[nczempin]

I created new issues for the three points above, #150, #151, #152

[nczempin]

So for this one it will be sufficient to draw a red indicator around the mass display.

[nczempin]

implemented in a32a6a5fae3054280194ab55c805b71d53dc2b0f