Add the textButton in Add Emission Screen

[PierreBresson]

In the Add Emission Screen, add the textButton from #315 and pressing it should open the Periodicity screen as a modal

NB: First to submit a valid PR, first to merge. Except if you already have merged some good code in this repo, I won’t assign any issue due to spam and beginners unable to complete a PR without a lot of help from my side.

[devtayls]

Hi, @PierreBresson I saw your repo listed in the hacktoberfest list. I'd like to help out a bit.

However, I'm having trouble building the application. Do you have a recommended guide to setting up the Metro Bundler Android / Iphone emulators?