Implement multi-display support

This commit implements multi-display support: the grid will be shown on
the display that currently has the cursor and can be moved through
screen borders.

Fixes #2

Load config from executable directory instead of working directory

Fixes #13

Read keynavrc from working directory

Note that this is technically non-standard behaviour (i.e. differs from
the original keynav).

Properly swap mouse button 2 and 3

The previous commit (fa83b90)
mistakenly only swapped the buttons for `doubleclick`.

Swap `click 2` and `click 3`

This makes the mouse buttons match the original keynav.

Recreate notification icon on TaskbarCreated

Fixes #11

Update resource file

Add a border around the grid to improve visibility and match keynav

Handle display resolution changes

Pass /SUBSYSTEM:WINDOWS to the linker

For x86 builds, DigitalMars's OPTLINK is used, which uses the CONSOLE
subsystem by default, even if WinMain is defined.