Implement hand tool

The logic for the hand tool is implemented in a separate project,
maintained at https://github.com/Rob--W/grab-to-pan.js

Integration notes
- Added toggle as an entry under the Secondary toolbar
- Added shortcut "h" to toggle hand tool (to-do: document this in wiki
  after merge). This shortcut is also used in Adobe's Acrobat Reader.

To-do: localizations for:
hand_tool_enable.title=
hand_tool_enable_label=
hand_tool_disable.title=
hand_tool_disable_label=

To-do (wish): persistence of hand tool preference, preferably a global setting.

secondaryToolbarButton-handTool.png created by Stephen Horlander <shorlander@mozilla.com>
Add support for horizontal scrolling in 'scrollIntoView' in ui_utils.js
Merge pull request #3684 from Rob--W/hand-tool

Implement hand tool
Dutch translations for the hand tool
Merge pull request #4007 from timvandermeij/hand-tool-nl

Dutch translations for the hand tool
Add hand tool strings to the Swedish l10n
Merge pull request #4008 from Snuffleupagus/hand-tool-l10n-sv

Add hand tool strings to the Swedish l10n
Hand tool translations for zh-TW
Merge pull request #4013 from xavier114fch/zh-TW

Hand tool translations for zh-TW
Comments:
Line 56-58: "Gegen den Uhrzeigersinn" is shorter and more common than "Entgegen dem Uhrzeigersinn" (http://de.wikipedia.org/wiki/Drehrichtung)

Line 60-63: "Hand-Werkzeug" is also used in Acrobat

Line 124,133,..: "PDF" is not a noun. Acrobat, Wikipedia use "PDF-Datei" (="PDF file") instead of "PDF".
The English translation sounds just as bad: "An error occurred while loading the PDF.", "The PDF is not fully loaded for printing."
Merge pull request #4018 from fkaelberer/l10nUpdatesDE

Update L10n for DE
Prevent the page number field from having a spinner (spin buttons) in mozilla now that it supports <input type=number>
Merge pull request #4020 from jwatt/master

Prevent the page number field from having a spinner (spin buttons) in mozilla now that mozilla supports <input type=number>
Update viewer.propertiesin l10n for zh-CN
Merge pull request #4024 from lpy/l10n-zh_cn

Update viewer.properties in l10n for zh-CN
Update zh-TW strings again. Pulled from latest mozilla-aurora
Merge pull request #4032 from moztw/1214-zhtw-v2

Update zh-TW again based on latest strings.
Fix typo in viewer.css
Merge pull request #4033 from timvandermeij/typo-fix

Fix typo in viewer.css
Update ja locale