Added the slider for fee selection

[masterchief164]

This PR is to add a new feature of the slider to choose transaction fees while sending bitcoins.

issue #125

[thunderbiscuit]

Good stuff! Thanks for taking this on. The only 2 small changes I would need before merging are:

1. The smallest value the slider should be able to set is 1.0.
2. This is a new feature, so your commit message should be something like Feat: Add slider for fee selection.

[thunderbiscuit]

Great little addition! Cheers 🔥