Fixed the plural handling in the Objects dock

The handling of different plural forms should be left to the Qt
translation system, rather than hardcoding special forms for the
singular case.
Removed a translation string by fixing a small inconsistency

Also removed the unneeded "Move Object To Layer", since it was
immediately overridden in ObjectsDock::updateActions anyway.
Updated the Dutch translation
Updated German translation
Updated all translation files again

I hope now we can enter string freeze.
Unified image layer rendering

The image layer rendering was different for each map renderer. It seems
to me that it is better to consider the image layer as simply
independent of the tile or object layer projections, which is what this
change does.

This change also implies that image layer position is now in pixels
rather than tiles. It is currently still not possible to change this
position in Tiled, so I don't expect that to affect anybody.

Also fixed the image layer item bounds, which resolves potential repaint
issues when changing image layer opacity or visiblity.

Closes #317
Cosmetic improvements to Tilesets dock and others

* Show selected tileset in tilesets menu, radio button style
* Show a large down arrow on the tilesets menu button
* Removed the space between the tab bar and the tileset view
* Removed the space between views and the tool bar below
Updated copyright year in About dialog

Also updated translation files for this change and marked the
translation as done in the German and Dutch translations.
Added Sébastien Burillard as French translator

Forgot that in commit 158918478fc9505e616d1d2ea08ed172f65c3980.
Updated Portuguese translation
Updated Spanish translation
Updated Russian translation
Made the actions in the Objects dock update to language change
Added manpages and desktop file
Updated French translation
Fixed Mac build on Qt 5

Due to missing headers in macsupport.mm.
Fixed spaces in debug message
Added a note about installing qt on Mac.
Updated Japanese translation
Initialize some members in the constructor

Makes cppcheck happier, and is a good idea anyway.