Add the wave system.

[CBenoit]

Should probably be implemented in abstract_level.gd.

During the wave:

• [x] spawn a given amount of enemies at various places (left, right, top).
• [x] a counter in the HUD should show the number of ennemies remaining.

At the end of the wave:

• [x] reward the player with money (carrots?)
• [ ] removes tiles damages (using the solids manager)
• [x] regenerate blocks outside of the base (function already implemented, just need to be called at the appropriate moment)
• [ ] maybe other things related to carrots (see https://github.com/CBenoit/cave-fortress/issues/8)
• [x] a counter in the HUD should show the remaining inter-wave time

[bloodre]

Added the wave scene allowing management of rift and enemies spawn. Also added a way to check the number of enemies attached to each individual rift. This allows to determine the end of the wave and display a progress bar. https://github.com/CBenoit/cave-fortress/commit/7ea5f5428be5f0fa9399d5363b3187d3fd699a6a

Issue: I added a function to the rift to clear the area around them. However, called from _ready(), a rift outside the base area still get surrounded.