Improve support for high-resolution stats

* This patch adds a new microsecond-resolution function call, LiGetMicroseconds(), to complement
the existing LiGetMillis(). Many variables used by stats have been updated to work at this
higher resolution and now provide better results when displaying e.g. sub-millisecond frametime stats.
To try and avoid confusion, variables that now contain microseconds have been renamed with a suffix
of 'Us', and those ending in 'Ms' contain milliseconds. I originally experimented with nanoseconds but it
felt like overkill for our needs.

Public API in Limelight.h:
uint64_t LiGetMicroseconds(void);
uint64_t LiGetMillis(void);
const RTP_AUDIO_STATS* LiGetRTPAudioStats(void);  // provides access to RTP data for the overlay stats
const RTP_VIDEO_STATS* LiGetRTPVideoStats(void);

Note: Users of this library may need to make changes. If using LiGetMillis() to track the duration of
something that is shown to the user, consider switching to LiGetMicroseconds(). Remember to divide by
1000 at time of display to show in milliseconds.

.gitignore

@@ -1,4 +1,5 @@
 .idea/
+.vscode/
 limelight-common/ARM/
 limelight-common/Debug/
 Build/