Bug Fixes / Better visual Feedback

Hi everyone, 

It's been rough weeks of development as i had to understand and script a way to finally get rid of the unwanted behavior where we could easily win... by just creating walls all around the character in order to be unkillable.  Now, when we do so the last wall we place shall get destroyed.

Along this major fix, I took the time to implement  other things : 

• Towers now can only be placed on blue areas, it means that you can build on the wall you create !
• Towers are now displayed in front of each other based on how close they are from the bottom of the screen
• Tower panel construction is forever visible, because why not ? 

Another Update that took me a lot of time to do is to show the avalaiblity of tower Upgrades. Basically  whenever we have enough gold to have a Tower Upgrade and that we unlocked it during the Choosing Upgrade Phase, a green "+" should appear to let us know if it's avalaible or not.