ViewWidget: Added mouse_down
ViewWidget: Moved in event and touch_event
Viewport: Use fabs instead of branch
Viewport: Implemented on-canvas drag
ViewWidget: Fixed glitching when drag is within the manhattan distance
Viewport: Implemented item hover cursors
dialogs::Connect: Adds shortcut keys for "Driver", "Serial Port" and "Scan for devices" GUI items.
MinGW: Fix a compile error due to missing std::isnan.

pv/view/viewport.cpp: In member function 'virtual void pv::view::Viewport::drag_by(const QPoint&)':
pv/view/viewport.cpp:85:24: error: 'isnan' was not declared in this scope
  if (isnan(drag_offset_))
                        ^
pv/view/viewport.cpp:85:24: note: suggested alternative:
In file included from pv/view/viewport.cpp:22:0:
[...]/usr/lib/gcc/i686-w64-mingw32.static/4.9.2/include/c++/cmath:632:5: note:   'std::isnan'
     isnan(_Tp __x)
     ^
Viewport: Removed broken using std::isnan
util: Added format_si_value
util: Added support for more SI prefixes
util: Added a space between the value and prefix
CursorPair: Added frequency indication

This fixes #502
View: Use the slider value for the v-offset
View: Added set_v_offset
Viewport: Mouse wheel scrolls vertically when the control key is pressed

This fixes #497