Providers configuration: fix #67 #116 #152 #222

[papjul]

Fix #67, fix #116, fix #152, fix #222

What has been done:

• [x] Display preferences in settings
• [x] Be able to edit preference value
• [x] Be able to save preference value
• [x] In API calls, use saved preference value instead of BuildConfig
• [x] Allow to reset value to default by editing text to "empty"

[papjul]

Ready for review, please review carefully, in case I made wrong copy/paste.