[Feature] Added emoji usage frequency support

[MajeurAndroid]

What is it ?

This PR adds - as an optional feature enabled by default - recent emojis sorted accordingly to their frequency of use. As opposite to the "last used" sorting currently in place.

The goal was to allow a sorting method that would take into account number of uses as well as how recent these uses are.

How it works ?

To explain this briefly, as incoming data we have dates at which emojis had been use, and as outcoming data we want a a value, that we called frequency, that we use to compare emoji against each other.

We compute that "frequency" for each recent emoji by following these steps:

• For each date of use, we compute the time elapsed from now, and divide it by a reference time period (24h, but it can be anything else).
• We then throw this value in a well chosen mathematical function that will give more importance for recent dates (2), and decreasing importance the older the dates get (tanganting 1 but never reaching it).
  Here is a graphical representation for 0 to 12 times our reference time (which is 24h):
  
• Because the mathematical function has been so well chosen, it gives values at least equals to 1, so we can multiply all the frequencies together to calculate the total frequency of this emoji.
• We then are able to sort every recent emoji based on previously calculated frequencues.

And now ?

The model for now is "as is", and is in test since 1 week, but may need a bit of testing by a larger audience. We can have a less stiff curve to give a bit less importance to "recentness" over number of uses.

Fixes #55