Fixes #2881, NullPointerExceptions when gc.com username is blank

- username, password cache was already removed
- returning empty pair on missing credentials
- cleaning up unneccessary getGcLogin() calls and null checks
fixes #3018 - show progress storing cache for edit personal note
Merge pull request #3046 from marco-jacob/fixStoreForEditPersonalNote

fixes #3018 - show progress storing cache for edit personal note
fixes #3021 - introduce named constants
Merge pull request #3047 from marco-jacob/introduceConstants

fixes #3021 - introduce named constants
Fixes #2988, New settings: Check and Save gc.com login not intuitive any more

- changing log-in info resets the login status and re-initiates log-in on return to MainActivity
Implements #17, Reload reCAPTCHA

Took layout and ideas raised by @culmor30
Moved reCAPTCHA key and challenge retrieval to the RecaptchaReceiver
Added slovenian translation - with fixes, squashed commit
Merge pull request #3048 from jeancaffou/master

Added Slovenian translations
Slovenian translation - fixed lint warnings and missing translations
Fixes #2918, Restore debug info for live map

Added url propagation to Viewport search in connector factory
Tuned debug info
Merge remote-tracking branch 'jeancaffou/master'
Slovenian - small post-merge fix
fix #3050 - progress for export field notes increases for every cache
Merge pull request #3056 from marco-jacob/fixProgressExportFieldNotes

fix #3050 - progress for export field notes increases for every cache
Create cache merging tests
fix #1851 - merge popup information
Fixes #3000: Basic members caches missing on map

Fixes #3000 - a second, much simpler fix (my first attempt being pull
request #3055).

The problem was that caches loaded from the nearby list were being
initialized with zoom level (max zoom level + 1), or 19. As far as I can
tell there was no useful purpose for that, but it was preventing caches
loaded from the live map at any zoom level from ever over-writing the null
coordinates. So I made caches initialize with zoom level -1 instead.
Finetuning fixes for #1851 and #3000 (cache merging)
change: make gpx export name more readable

* use only date instead of date and time
* format date using dashes
* append _2, _3, ... with further exports