Make Ninja class more playable (maybe buff)

[Kaffeine]

Suggested changes:

• Make the ninja invincible for hammers during the slash (the split second of fast attack swing)
• Change the ninja rewards to a constant list
• Make the ninja targets individual (instead of global target with global cool down) #51

Ninja rewards (v2):

1. +1 dmg, +1 velocity
2. invisibility grendes (0.75/1.5 sec duration?)
3. faster ammo regen
4. increased flash AoE
5. increased invisibility duration (1/2.5 sec?)

The suggested rewards list (an example for the first approximation):

1. +1 slash dmg
2. extra velocity (worth like 2-3 current velocity buffs)
3. faster ammo regen
4. extra flash grenade AoE
5. +1 slash in the air

[Kaffeine]

@Intramint please correct me if I wrong.

[Kaffeine]

Currently (newly) implemented rewards list:
Level 1: Sword strength increased (damage +1)
Level 2: Sword velocity increased (2nd from the buffs list)
Level 3: Ammo regeneration accelerated (Default interval is 15 sec, reduce to 11 sec, also +2 max ammo)
Level 4: Flash grenade radius increased (Default radius is 8 tiles, increase to 11)
Level 5: An extra ninja swing given (Default inf_ninja_jump 2, give an extra jump for this ninja)

[Kaffeine]

Side effects: it turned out that the collisions are not calculated properly. With the velocity buff there is a change to get through.
Rework this code to fix the bug: https://github.com/InfectionDust/teeworlds-infclassR/blob/8f622e1d2f9eb7b6041f603f0e0484f826e44d54/src/game/server/entities/character.cpp#L305-L356

[Kaffeine]

@InfectionDust/scientists and @InfectionDust/engineers, let's the flame begin.

[teoman002]

Let ninja have another slash, if he hits a zombie mid-air. This will be hilarious and Ipoopi will love to abuse it. xD