Fix "CursorVisible bugs"

When we enter the modal resize loop on Windows with ClipCursor set, we
cause a feedback loop where every resize causes the cursor to move and
every move causes a new resize. To fix this, we need to ungrab the
cursor when we are enter the modal loop.
Enabled KeyDown and KeyUp events in NativeWindow.
Implemented KeyDown and KeyUp events for Sdl2NativeWindow.
Implemented KeyPress event for Sdl2NativeWindow.
Add test for new text input events
Made DPI-awareness configurable (issue #6)

It is now possible to indicate that an application is not DPI-aware. In
that case, OpenTK will let the operating system handle DPI scaling. This
results in worse visuals (pixel doubling) but allows non DPI-aware
applications to continue working.
High-resolution mode is enabled by default
DPI-scaling should affect resolution

The correct resolution will now be reported depending on whether the
application is DPI-aware.
Fixed centering for non DPI-aware applications
Workaround for #5

WS_CLIPCHILDREN and WS_CLIPSIBLINGS appear to cause flickering on
specific video cards. OpenGL appears to work correctly without these, so
we'll disable them to return to OpenTK 1.0 behavior.