Issue 1401: Text color for white-theme buttons.

Currently the buttons do not have an explicit text color. This works if
the device default theme has a proper color for the text by default, but
fails if the default color is not readable.

This commit makes the color explicit and therefore makes it the same on
all devices.
Don't center-align cards.
Merge pull request #12 from ntsp/Issue635

Issue 635 - Don't center-align cards in global style rule
Merge branch 'v2.1-iss1401' of git://github.com/flerda/Anki-Android into v2.1-dev
Options group management
Merge pull request #11 from ntsp/Add/Remove/Reset-Options

Options group management
Reorder cards according to default group order when deleting options group.
Merge pull request #14 from ntsp/conf-del-sorting

Reorder cards according to default group order when deleting options group
Fix sync issue with multiple users on android 4.2 (issue 1609)
Merge pull request #15 from Prom42/storage-fix

Issue 1609: Fix sync issue with multiple users on android 4.2 tablets.
Back button on tablet congrats screen should return to overview
Merge pull request #17 from hssm/Issue1488

Back button on tablet congrats screen should return to overview.
Set language from settings on start (Issue 1309)
Merge pull request #16 from Prom42/multilanguage

Issue 1309: Set language from Preferences on start of ankidroid
Custom preferences for number range and steps input.

Also catch number format exceptions in custom study input to avoid
crashing the application.
Deck options XML formatting
Merge pull request #19 from hssm/options-validation

Input validation with custom preferences.
Fix for Issue 1598: Filtered deck crash when revealing answer
Merge pull request #20 from Prom42/fixing

Fix for Issue 1598: Filtered deck crash when revealing answer.
Update preference constructors.

Delegate to the most general constructor and follow the guidelines in
  http://developer.android.com/reference/android/view/View.html#View(android.content.Context)
and:
  http://stackoverflow.com/questions/8113621/which-constructor-to-be-called-for-view