Allows line tool to snap to angles on shift key.

Lets any shape tool implement shift features, snap code from paint.net
3.36.
Added line snapping to tooltip for line tool.
[Fixes bug #852385] Fix crash with Splatter brush of size 1
Fix selection with constraint and bug if origin coord is less than end
[Fixes bug #852770] Make Ctrl+Scrollwheel work with Caps Lock and Num Lock

Instead of checking that Ctrl is the only modifier set, we now check
that Ctrl is one of the modifiers set and that Shift/Alt are not pressed
as well.
This allows Ctrl+Scrollwheel to work when other modifiers (such as Caps Lock
and Num Lock) are set.
[Fixes bug #850806] Remove AC_PREFIX_DEFAULT([/usr])
Fixed the build system and also added the pinta.pc.in file

Detailed explanation of the changes are:
* Added pinta.pc.in file which will be used to build extensions for pinta since Pinta now has Mono.Addins support
* Added pinta.pc in configure.ac file AC_CONFIG_FILES
* Removed the entries for 'Tarball', 'System.ComponentModel.Composition', 'readme.txt' from EXTRA_DIST in ./Makefile.am
* Added 'pinta.pc.in' in EXTRA_DIST in ./Makefile.am
* Added 'pinta.pc' in distclean section of ./Makefile.am
* Created MACRO named 'PINTA_ASSEMBLY_PC' in ./Makefile.am
* Added automake magic variables in ./Makefile.am
* Fixed BINFILES macro by removing 'Extensions' path entry for Pinta.Effects.dll and Pinta.Tools.dll
* Removed 'System.ComponentModel.Composition.dll' from BINFILES macro
* Added pinta.pc.in file which contains the link to all the assemblies present with Pinta
Added pinta.pc in .gitignore and added readme.md to Makefile.am EXTRA_DIST
Updated autogen.sh to find intltool and initialize it
Added Makefile reference in configure.ac so that Makefile can be generated for xdg directory and also change pinta.desktop to pinta.desktop.in
Got the basic version of localized desktop file working. The values are not getting substituted by atlest .desktop file is getting created
Merge pull request #4 from manish/master

Fixed the build system due to which make was working but everything else (example tarball generation) was broken. Apart from fixing broken build system, pinta.pc.in was added to allow third-party Extensions to be built
Made changes to messages.pot so that it does include the translatable strings

Next step to fix. The names of the language pot files
Localized the X-GNOME-FullName too.
Renamed all the messages-<locale>.po to <locale>.po as per the standard convention. Also note that this fixed the breaking .destop file localization
[Fixes bug #731461] Enlarge the canvas size when pasting from clipboard
Merge pull request #5 from manish/master

Fix Launchpad Bug #660617 to make .desktop file translatable
Fixed the pinta.pc.in file to use @libdir@ instead of @prefix@/lib
Merge pull request #6 from manish/master

Fixed the filenames of locale .po files and fix the small issue in .pc.in file
Remove temporary test translations from br.po