Improve File Change Notification for slow medium

[ProgerXP]

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Since file monitoring is on the main thread (WM_TIMER), slow FindFirstFile() makes the program almost non-responsive. This is common with network shares outside of the local network. A simple solution is to disable monitoring or increase polling interval, but a better approach would be to move monitoring to another thread. I have briefly checked the code and it seems easy to convert WatchTimerProc() into a thread function.

Related to #241.

[cshnik]

Done.

[ProgerXP]

I have started to see notifications when file is saved because of Save On Lose Focus (#164). This is extremely rare but I am confident there is a bug in the implementation.

Update: #164 might be causing this more often but this definitely occurs even with it disabled.

[cshnik]

Fixed.