gui: Look for icon file in current dir on Windows

Similar to the previous change for *.ui.  This makes the app's icon show
up on the taskbar and on the window menu (alt-space).

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
buildsys: Add "make winpkg" target

This creates a zip file with all EXEs and DLLs needed to run stoken and
stoken-gui on Windows.  Experimental.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
README: Change to markdown format; update with GTK+3 and Windows info

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>