Add achievement system

[4Kaylum]

This is one of the original ideas I had to add in to Flower, but now I feel like I can start tracking enough stats on this to make it actually like... work.

Ideas for achievements:

• Getting 1 of every type of flower
• Getting N of every type of flower
• Killing N flowers
• Trading N flowers
• Reviving N flowers
• Immortalising N flowers
• Keeping a plant alive for N time

I'm happy to get suggestions on this - I'm still not entirely convinced it's a good idea.

[MakiOphelia]

Oooo I have a few suggestions/ideas here

Achievements:
Total XP Gain
Total Consecutive watering (at least once per hour) might be probably too complicated

Leveling
Maybe like you gain another type of experience? Or you get 150% exp but the 50% goes to your level like you level up with titles like Greenhorn, novice florist, etc

[4Kaylum]

Total consecutive watering might be a lil difficult to implement, but total exp gain would be fine if people didn't mind starting from when I implement it without backdating.

The achievement branch has some untested stats tracking of all the things I suggested myself.