[pki] add both SHA-1 and SHA-256 digital signatures

* "[Grumble] Stupid Microsoft... [Grumble] Couldn't push updated PKI algorithms
   to Windows platforms... [Grumble] Even if their lives depended on it..."

[misc] add support for a 'test' release channel

* This gets enabled only for TEST builds
* Also flesh out the sample rufus.ini

[ui] fix shutdown prevention issues

* Also minor UI improvements for High DPI
* Part of issue pbatard#676

[ui] adaptive height resizing of SHA-256 checksum control

[vhd] fix an issue when extracting WIM with 7z

* 7z has a quirk in that one MUST specify the image index
  when a WIM has multiple images, but the same index MUST
  be omitted if there is only one.
* Because of this, Windows 7 Enterprise images reported a
  (nonfatal) extraction error when trying 7z.
* Closes pbatard#680
* Also fix a typo in registry.h

[loc] update Dutch translation to latest

[loc] update Croatian translation to latest

[ms-sys] upgrade ms-sys to v2.5.2

[loc] update Slovak translation to latest

[ui] add drag and drop support

* This feature is only available for Vista or later
* Closes pbatard#316
* Closes pbatard#668