New plants and bug fixes!

Introducing: Farm levels!  Now as you harvest plants you will gain experience and unlock new crops for your farm.  The crops get more and more lucrative as you level up, with varying growth times.

* Fixed bug that would disappear seeds if you held them while clicking "quick plant"

* Fixed a bug that would allow you to plant infinite seeds after stacking them in your inventory

* Updated seed packet, farm plot, portrait frames, and plant grow sprites

* Added farm levels and new seed unlocks

* Added visual feedback when purchasing, selling and harvesting